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The role of hoods on a lens.
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PostPosted: Sun Apr 10, 2011 11:23 pm    Post subject: Reply with quote
Rodinal wrote:
Is this nescessarily so? By keeping the law of diminishing returns in mind, I suspect that using a medium long hood on a long tele you block out so much of the stray light that the rest doesn't make any difference. By that logic, it makes sense to own hoods up to, say, 100-200mm but after that there's nothing to be gained by using longer ones to match the focal length.


The definition of "diminishing returns" depends on your priorities. Wink Unless weight & size wins over image quality, I wouldn't skimp on hoods for the big lenses.


The longer the focal length, the narrower the angle of view, and the less effective an under-sized hood will be. While a shorter hood may block the direct overhead sun and the "majority" of the stray light, the rest of the stray light can still play havoc, especially when there are bright lights outside the captured frame but still inside the view of the lens.


From my testing, for telephoto hoods, the longer the better. Here is a comparison shot, with the camera settings locked and nothing changed except the hood:





PostPosted: Mon Apr 11, 2011 12:01 am    Post subject: Reply with quote
If your lens has poor coating and/or internally reflecting surfaces, hood can help you as long as the sun or other strong light source do not make it into the frame. When they do, no hood can help you.

If you lens has a very good coating, hoods do not really improve contrast, but they can help you spare the occasional ghosts.

If your lens has supreme quality coating, like the Contax Distagon 1.4/35, you can pretty much live hoodless in any situations (at least, I do, and I still have to get a flaring picture after many thousands of shots with that lens).

Of course, asking if it's better multicoating alone or multicoating with hood, is a bit like asking if it's better to have a great dinner or a great dinner + a great dessert... if you know what I mean. Rolling Eyes
(obviously, it's the dinner you get the real mileage from, but the dessert only adds, albeit little)


PostPosted: Mon Apr 11, 2011 12:10 am    Post subject: Reply with quote
Re: Scheimpflug's test shots. If people cannot see the difference between those two shots, then they are flat out blind! I like the latter. It definitely makes up for PP later.

I think it is awesome old lenses came with pre-built lens hoods. My 400 mm has a very long lens hood. In a way, a lens hood may act to sort of stop down the lens a bit?

It is interesting the effect on the bokeh, if you will....kind of like the difference between a wide-open aperture on a fast lens or not so fast lens and a more narrow aperture.

This leads to another question. Do lens hoods affect bokeh?


PostPosted: Mon Apr 11, 2011 2:34 am    Post subject: Reply with quote
newton wrote:
Re: Scheimpflug's test shots. If people cannot see the difference between those two shots, then they are flat out blind! I like the latter. It definitely makes up for PP later.

Yes, the results are quite distinct! Wink

The difference is also very apparent when you compare the histograms:





The shot with the larger hood has much deeper shadows, since they weren't artificially lifted by the scattered light, and the overall histogram spread is much wider. I think there are a lot of cases where telephoto shots really seem to need a lot of post-processing, where in reality, they just needed a decent hood at the start. Wink


For reference, the hood on the second shot was as long as the lens. Shocked Not especially practical for regular use, but it was a good test to show the benefits of a good hood! Wink


PostPosted: Mon Apr 11, 2011 5:36 pm    Post subject: Reply with quote
newton wrote:
In a way, a lens hood may act to sort of stop down the lens a bit?

It is interesting the effect on the bokeh, if you will....kind of like the difference between a wide-open aperture on a fast lens or not so fast lens and a more narrow aperture.

This leads to another question. Do lens hoods affect bokeh?

No, a hood doesn't have the effect of stopping down. All that you are seeing is higher contrast in the shot with the hood, because there is no veiling flare. Increased contrast can make the bokeh look a bit less soft, but a hood changes nothing about the out of focus rendering of the lens.

An alternative view is that a hood is an aperture that prevents stray light from entering, but you normally don't want that light to enter the lens anyway because it masks image information: the shot without hood has a smaller dynamic range due to flare. Flare can be used creatively though: http://www.fredmiranda.com/forum/topic/891645/22#9474816


PostPosted: Mon Apr 11, 2011 6:30 pm    Post subject: Reply with quote
Scheimpflug wrote:


For reference, the hood on the second shot was as long as the lens. Shocked Not especially practical for regular use, but it was a good test to show the benefits of a good hood! Wink


Always nice to see hard data - it beats an internet rumour any day of the week Smile

Could you share what lens you used? Is this a modern Zeiss or something more humble (and more prone to flare) ?

Björn


PostPosted: Tue Apr 12, 2011 10:05 am    Post subject: Reply with quote
I have a not yet mentioned use for a hood:
I made a tulip hood for my Rokkor 58mm f/1.2 that adds a bit vignetting if the lens is used wide open.
This vignetting is part of my "style" that I use for some images. Sometimes I add some more vignetting in Lightroom.


PostPosted: Fri Apr 29, 2011 5:03 am    Post subject: Reply with quote
Rodinal wrote:
Always nice to see hard data - it beats an internet rumour any day of the week Smile

Could you share what lens you used? Is this a modern Zeiss or something more humble (and more prone to flare) ?

Björn


Certainly not modern, but not necessarily humble either... The lens in those tests was an 1880's-era Rapid Rectilinear made by Taylor, Taylor & Hobson. Cool

It is an uncoated lens designed for large format work (half-plate?), so it certainly would be more prone to flare than most other lenses.









I have a thread about it here:
http://forum.mflenses.com/1880s-brass-taylor-taylor-and-hobson-rapid-rectilinear-t30299.html


PostPosted: Sat Apr 30, 2011 3:10 pm    Post subject: Reply with quote
Fascinating! Wow!!


PostPosted: Sat Apr 30, 2011 11:15 pm    Post subject: Reply with quote
visualopsins wrote:
AhamB wrote:
visualopsins wrote:
Petal hoods are an attempt to even the shading over long and short dimensions and to let more light in the corners to reduce vignetting effects. Imho, these 'benefits' are 'gained' at the expense of less reduction of out-of-view lighting, less reduction of flare and ghost.


That would only be true if the petal hood is shorter in the corners than a circular hood with the same diameter. Circular hoods need to have a larger diameter to be as effective as petal hoods, and then they still leave gaps for stray light to strike the front lens (unless you oversize the hood and accept vignetting).


Thanks! I have changed my mind.

I understand petal hoods are smaller than round hoods for same effect.

Also, petal hoods are more effective than round hoods because petal hood shading extends into the entrance pupil circle, vignetting top & bottom of exit pupil image circle, unused by 3:2 format -- extraneous light entering the lens is reduced more than round hood.


You should'nt have changed your mind because you were right the first time...A petal hood can never be more effective than a cylindrical hood because it only shades the lens from four directions at any one time, not from 360 degrees simultaneously as a cylindrical hood does. Wink


PostPosted: Sun May 01, 2011 1:14 am    Post subject: Reply with quote
DSG wrote:
visualopsins wrote:
AhamB wrote:
visualopsins wrote:
Petal hoods are an attempt to even the shading over long and short dimensions and to let more light in the corners to reduce vignetting effects. Imho, these 'benefits' are 'gained' at the expense of less reduction of out-of-view lighting, less reduction of flare and ghost.


That would only be true if the petal hood is shorter in the corners than a circular hood with the same diameter. Circular hoods need to have a larger diameter to be as effective as petal hoods, and then they still leave gaps for stray light to strike the front lens (unless you oversize the hood and accept vignetting).


Thanks! I have changed my mind.

I understand petal hoods are smaller than round hoods for same effect.

Also, petal hoods are more effective than round hoods because petal hood shading extends into the entrance pupil circle, vignetting top & bottom of exit pupil image circle, unused by 3:2 format -- extraneous light entering the lens is reduced more than round hood.


You should'nt have changed your mind because you were right the first time...A petal hood can never be more effective than a cylindrical hood because it only shades the lens from four directions at any one time, not from 360 degrees simultaneously as a cylindrical hood does. Wink


The petals are longer than cylindrical hood can be -- petal indentation to lens distance is about the same as cylinder hood is deep. Petal hood shades more than cylinder hood Wink


PostPosted: Sun May 01, 2011 2:32 am    Post subject: Reply with quote
visualopsins wrote:
DSG wrote:
visualopsins wrote:
AhamB wrote:
visualopsins wrote:
Petal hoods are an attempt to even the shading over long and short dimensions and to let more light in the corners to reduce vignetting effects. Imho, these 'benefits' are 'gained' at the expense of less reduction of out-of-view lighting, less reduction of flare and ghost.


That would only be true if the petal hood is shorter in the corners than a circular hood with the same diameter. Circular hoods need to have a larger diameter to be as effective as petal hoods, and then they still leave gaps for stray light to strike the front lens (unless you oversize the hood and accept vignetting).


Thanks! I have changed my mind.

I understand petal hoods are smaller than round hoods for same effect.

Also, petal hoods are more effective than round hoods because petal hood shading extends into the entrance pupil circle, vignetting top & bottom of exit pupil image circle, unused by 3:2 format -- extraneous light entering the lens is reduced more than round hood.


You should'nt have changed your mind because you were right the first time...A petal hood can never be more effective than a cylindrical hood because it only shades the lens from four directions at any one time, not from 360 degrees simultaneously as a cylindrical hood does. Wink


The petals are longer than cylindrical hood can be -- petal indentation to lens distance is about the same as cylinder hood is deep. Petal hood shades more than cylinder hood Wink


Thats incorrect...Cylindrical hoods can be much longer than petal hoods so Cylindrical hoods shade more than Petal hoods. My 77mm Sonia brand metal lenshood for my Sigmatel 135mm f1.8 is 3" deep...As far as I am aware there are no 3" deep petal hoods.


PostPosted: Sun May 01, 2011 3:56 am    Post subject: Reply with quote
DSG wrote:
visualopsins wrote:
The petals are longer than cylindrical hood can be -- petal indentation to lens distance is about the same as cylinder hood is deep. Petal hood shades more than cylinder hood Wink


Thats incorrect...Cylindrical hoods can be much longer than petal hoods so Cylindrical hoods shade more than Petal hoods. My 77mm Sonia brand metal lenshood for my Sigmatel 135mm f1.8 is 3" deep...As far as I am aware there are no 3" deep petal hoods.


Sorry, but visualopsins has it right. Wink

You can verify it for yourself if you want. Just put that deep hood on your Sigmatel, and then look through the viewfinder. Hold a finger out just at the edge of the hood on one of the corners of the frame (such as the 10 o'clock or 2 o'clock position), then move it away (straight away, parallel to the lens axis), and see how far you can move it before it shows up in the frame. Now, put your finger directly centered on the top or bottom of the hood (12 o'clock or 6 o'clock), and do the same. See how much further you have to go before it becomes visible? That is the extra area a petal hood will shade, which a standard circular hood will not.


... and deep petal hoods *do* exist for telephoto lenses... they just aren't as common. Here are two results from a quick Google search:


http://masterchong.com/v2/article/sony-sal70300g-lens-review-and-sample-photos.html


http://www.dpreview.com/lensreviews/canon_70-200_2p8_is_usm_ii_c16/page3.asp

The Canon hood there is listed by dpreview as being 95mm (3.75") deep. Wink I don't have any measurements for the Sony hood, but it appears to be quite a bit deeper... at least, relative to the diameter.


PostPosted: Wed May 04, 2011 5:21 am    Post subject: Reply with quote
In a benefit/lenght wouldn't be square shades the most effective?


PostPosted: Wed May 04, 2011 11:06 am    Post subject: Reply with quote
Eugen Mezei wrote:
In a benefit/lenght wouldn't be square shades the most effective?


Only if the lens elements were square too...But they are round so a round hood is more suitable.
And a cylindrical hood is always best Wink


PostPosted: Wed May 04, 2011 6:01 pm    Post subject: Reply with quote
DSG wrote:
Eugen Mezei wrote:
In a benefit/lenght wouldn't be square shades the most effective?


Only if the lens elements were square too...But they are round so a round hood is more suitable.
And a cylindrical hood is always best Wink

Sorry, but the exact opposite is true. The sensor is rectangular so the hood may be rectangular. The top and bottom petals of petal type hoods are longer than the side ones for this reason too.
Cylindrical hoods are least efficient -- that's something that was established earlier on in this thread. They can only adequately shade the lens for the corners, and will because of their geometry leave gaps for stray light to enter in the places where the hood doesn't extend far enough. Petal hoods have extensions in these places. It's as simple as that.

See: http://toothwalker.org/optics/lenshood.html
Scroll down to the section "Shape of the hood" to see an illustration of what I'm talking about.


PostPosted: Wed May 04, 2011 7:00 pm    Post subject: Reply with quote
DSG wrote:
Eugen Mezei wrote:
In a benefit/lenght wouldn't be square shades the most effective?


Only if the lens elements were square too...But they are round so a round hood is more suitable.
And a cylindrical hood is always best Wink


Yes, on Petzwals Kamera. Very Happy


PostPosted: Wed May 04, 2011 7:06 pm    Post subject: Reply with quote
AhamB wrote:
DSG wrote:
Eugen Mezei wrote:
In a benefit/lenght wouldn't be square shades the most effective?


Only if the lens elements were square too...But they are round so a round hood is more suitable.
And a cylindrical hood is always best Wink

Sorry, but the exact opposite is true. The sensor is rectangular so the hood may be rectangular. The top and bottom petals of petal type hoods are longer than the side ones for this reason too.
Cylindrical hoods are least efficient -- that's something that was established earlier on in this thread. They can only adequately shade the lens for the corners, and will because of their geometry leave gaps for stray light to enter in the places where the hood doesn't extend far enough. Petal hoods have extensions in these places. It's as simple as that.



No it is'nt, thats plain wrong, because unlike a petal hood, a cylindrical hood has NO gaps to let light in! Light can only enter from the front of a cylindrical hood but with a petal hood it can enter between the petals!
Besides, petal or cylidrical, if you want more shade you simply use a longer hood...When opaque vignetting occurs at the corners of the frame you have gone too far and you will need a slightly shorter hood...Its a simple as THAT! Wink


PostPosted: Wed May 04, 2011 7:11 pm    Post subject: Reply with quote
DSG wrote:
No it is'nt, thats plain wrong, because unlike a petal hood, a cylindrical hood has NO gaps to let light in! Light can only enter from the front of a cylindrical hood but with a petal hood it can enter between the petals!
Besides, petal or cylidrical, if you want more shade you simply use a longer hood...When opaque vignetting occurs at the corners of the frame you have gone too far and you will need a slightly shorter hood...Its a simple as THAT! Wink
(i) Obviously a rectangular sensor/film doesn't use all the light from the image circle a lens projects onto the sensor/film-plane.

(ii) This "excess" light can be shielded off with the petals on the lens hood. So a petal lens hood, if designed correctly, will shield more unnecessary light than a circular hood of the same base-length.

Makes sense now?


PostPosted: Wed May 04, 2011 8:55 pm    Post subject: Reply with quote
DSG wrote:

No it is'nt, thats plain wrong, because unlike a petal hood, a cylindrical hood has NO gaps to let light in! Light can only enter from the front of a cylindrical hood but with a petal hood it can enter between the petals!


That would be only true if the petal hood is as long as the equivalent cylindrical (more correctly conic) one.

Quote:

Besides, petal or cylidrical, if you want more shade you simply use a longer hood...


This is limited by when the hood will begin vignetting. Maybe when using a conical hood with exactly the same angle as the angle of view of the lens you could extend the hood at infinity. But ever than you will have light that hits not the film/senzor area (that is not round) but is bouncing around the film chamber.


Quote:

When opaque vignetting occurs at the corners of the frame you have gone too far and you will need a slightly shorter hood...Its a simple as THAT! Wink


First thing correct, second not. It is not at all so simple.

Don't think about the petal hood as being the same lenght as the round one. Think about the usable light rays as building a pyramid with the top being the film/senzor square. Using a cylindrical hood you can extend this as long as it hits the 4 corners of this pyramid. Extending further the cylinder will more and more cut of of this corners. The only way to make the hood longer without vignetting is to cut the 4 pieces out of it along the corners of the pyramid. So the petal hood is always longer as the longst non-vignetting round hood.
Now you say light can enter the "holes" (cutouts) but that is exactly what you want. This light is part of the aformentioned pyramid, so you want this light to hit the film/senzor square.
What you don't want is the light entering the four arches built between the 4 meeting points of the round hood with the pyramid. This arch shaped gaps between pyramid and the outmost circle of of the round hood is light you want to block as it isn't directly received by the film/sensor but reaches this by reflexion and obviously produces flare.

The first step to get the catch with the petal hoods is to realise they are longer than a round one and to think about them as a vignetting round hood with the vignetting parts (if you had vignetted images by mistake you could see that the corners get first blackened) cut out.

As said before, with circular film it would be much simpler. You could use a round hood and have maximum efectiveness.


PostPosted: Wed May 04, 2011 11:32 pm    Post subject: Reply with quote
Eugen Mezei wrote:
DSG wrote:

No it is'nt, thats plain wrong, because unlike a petal hood, a cylindrical hood has NO gaps to let light in! Light can only enter from the front of a cylindrical hood but with a petal hood it can enter between the petals!


That would be only true if the petal hood is as long as the equivalent cylindrical (more correctly conic) one.

Quote:

Besides, petal or cylidrical, if you want more shade you simply use a longer hood...


This is limited by when the hood will begin vignetting. Maybe when using a conical hood with exactly the same angle as the angle of view of the lens you could extend the hood at infinity. But ever than you will have light that hits not the film/senzor area (that is not round) but is bouncing around the film chamber.


Quote:

When opaque vignetting occurs at the corners of the frame you have gone too far and you will need a slightly shorter hood...Its a simple as THAT! Wink


First thing correct, second not. It is not at all so simple.

Don't think about the petal hood as being the same lenght as the round one. Think about the usable light rays as building a pyramid with the top being the film/senzor square. Using a cylindrical hood you can extend this as long as it hits the 4 corners of this pyramid. Extending further the cylinder will more and more cut of of this corners. The only way to make the hood longer without vignetting is to cut the 4 pieces out of it along the corners of the pyramid. So the petal hood is always longer as the longst non-vignetting round hood.
Now you say light can enter the "holes" (cutouts) but that is exactly what you want. This light is part of the aformentioned pyramid, so you want this light to hit the film/senzor square.
What you don't want is the light entering the four arches built between the 4 meeting points of the round hood with the pyramid. This arch shaped gaps between pyramid and the outmost circle of of the round hood is light you want to block as it isn't directly received by the film/sensor but reaches this by reflexion and obviously produces flare.

The first step to get the catch with the petal hoods is to realise they are longer than a round one and to think about them as a vignetting round hood with the vignetting parts (if you had vignetted images by mistake you could see that the corners get first blackened) cut out.

As said before, with circular film it would be much simpler. You could use a round hood and have maximum efectiveness.


Very good answer Eugen...I now understand why the petal hood has petals and why. I still think they look ugly compared to cylidrical (not conical) hoods but now I can see their advantages, at least for use with wider lenses. Thats also explains why my Sigma 14mm f3.5 ultrawide has a built in petal hood.


PostPosted: Wed May 04, 2011 11:42 pm    Post subject: Reply with quote
Surely a useless precisation for most, but possibly useful for newbies: with a petal hood, you need to make sure that it's correctly aligned with the camera, otherwise the maximum effectiveness of the design turns into a poor protection. With dedicated petal hoods that use bayonet, this is also a useless remark, but important for those who use generic screw-in petal hoods.


PostPosted: Thu May 05, 2011 12:32 am    Post subject: Reply with quote
To continue the tips series begun by Orio:
When mounting a screw-in square hood you need to assure it sits paralell to the edges of the film/senzor frame.
I used to do this holding camera (with lens on it) and hood in the hand and adjusting by eye. I was never shure if it is or not fixed really paralell.
Until I read a tip saying to put camera and screwed in hood onto a table and screw the counterring just next to this. The under side of the hood and the camera bottom lying on the same surface will assure they are paralell. Much easier than eyeballing it.


PostPosted: Thu May 05, 2011 2:58 am    Post subject: Reply with quote
AhamB wrote:
DSG wrote:
Eugen Mezei wrote:
In a benefit/lenght wouldn't be square shades the most effective?


Only if the lens elements were square too...But they are round so a round hood is more suitable.
And a cylindrical hood is always best Wink

Sorry, but the exact opposite is true. The sensor is rectangular so the hood may be rectangular. The top and bottom petals of petal type hoods are longer than the side ones for this reason too.
Cylindrical hoods are least efficient -- that's something that was established earlier on in this thread. They can only adequately shade the lens for the corners, and will because of their geometry leave gaps for stray light to enter in the places where the hood doesn't extend far enough. Petal hoods have extensions in these places. It's as simple as that.

See: http://toothwalker.org/optics/lenshood.html
Scroll down to the section "Shape of the hood" to see an illustration of what I'm talking about.


Rectangular hoods Wink Idea Rectangular hoods with petals!


PostPosted: Thu May 05, 2011 9:23 am    Post subject: Reply with quote
Interesting discussion going on here though I can't say I understand all that were said. It really trigger me thinking as I generally don't use hood on my lens.
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