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minolta 250 mirror
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PostPosted: Wed Feb 25, 2015 7:00 am    Post subject: Reply with quote
So in conclusion, 72mm diameter lenses are better optically than 67mm diameter lenses? But losing the advantage of compactness?


PostPosted: Wed Feb 25, 2015 7:27 am    Post subject: Reply with quote
I won't draw a conclusion by filter size like that.

I have a few mirror lenses: this minolta, tamron 55BB and canon fd.
They all good but with different properties:
minolta: very compact and light, last to go. no problem to use with sony a7r 36Mpix.
tamron: 500/8 is very good for close range, but focus throw is too short in long range that makes it hard to use (adaptall2)
canon fd: 500/8 better focus throw in mid-long range, biggest and heaviest of the mirror lenses i have.

tamron with TCx2 like 01F or 200F becomes 1000mm still usable; resolution is a bit washed out with TC.


PostPosted: Wed Feb 25, 2015 7:38 am    Post subject: Reply with quote
edri wrote:
So in conclusion, 72mm diameter lenses are better optically than 67mm diameter lenses? But losing the advantage of compactness?

No. It is the design that matters.


PostPosted: Wed Feb 25, 2015 8:22 am    Post subject: Re: minolta 250 Reply with quote
For that design matters.

paulhofseth wrote:
the small Minolta 250 is fine. Compact and does what it promoises to do.

If you are buying, do remember to get it with the clear glass rear filter in place-. The other filters are nice to have, but one must be there in order for it to work properly.

p.


Very useful information for me. After reading this post I remembered that I have read something similar in a book, but I quickly passed over it. Now I found the paragraph: "... that mirror lenses are normally designed to be used with rear filters, and they must have a filter in the light path, even it is only a clear UV filter ... what is really inconvenient is if you buy a second-hand lens and the filters are not supplied... "

I had a lens which I considered mediocre, the Makinon 500/8. I used it without a filter. I mount the rear 1A filter, thrown somewhere in the closet, until today. After a few handheld test shots, surprise, I found that my mediocre lens has become a good lens Smile. Pictures are much clearer than without filter.

Why? What is the explanation that for mirror lenses the rear filter must be mounted?


PostPosted: Wed Feb 25, 2015 9:11 am    Post subject: rear filter Reply with quote
This is a similar effect as with lots of glass in front of a digital sensor. Light rays crossing from one medium to another get bent. So a design made to correct for a glass mounted at the rear of the lens will have a different performance when those two transitions -in and out of the filter- are absent. The size of the effect will depend on the entry angles concerned and the refraction and dispersion qualities of the glass, even if the surfaces to be crossed are perfectly plane parallell.

I have not tried the Minolta without the rear filter, so I do not know how significant it is. I do not have the original documentation to hand right now, but a paraphase from memory is that "the filters that came with it should be used at all times to avoid image deterioration".

p.


PostPosted: Wed Feb 25, 2015 9:17 am    Post subject: Reply with quote
Thanks for posting. A lens that I do not enjoy using it starts to become a new favorite Smile


PostPosted: Wed Feb 25, 2015 9:36 am    Post subject: Reply with quote
My experience is that all the mirror lens I tried have no negative impact without the rear filter and many of them does work better without the the rear filter. If you want to know more, read through the first page of this thread http://forum.mflenses.com/sensor-stack-size-interesting-reading-for-legacy-lens-users-t66629.html .


PostPosted: Wed Feb 25, 2015 11:38 am    Post subject: Reply with quote
Maybe for expensive lenses as you have this effect is not obvious and it is observable for ordinary lenses.

Anything else about mirror lenses from the same "The lens book" :

"... typically the effective aperture of a mirror lens is a stop less than the marked aperture: f/11 for f/8; f16 for f11."


PostPosted: Wed Feb 25, 2015 1:36 pm    Post subject: Reply with quote
edri wrote:

Anything else about mirror lenses from the same "The lens book" :

"... typically the effective aperture of a mirror lens is a stop less than the marked aperture: f/11 for f/8; f16 for f11."

and the main caused for lost of light is the blockage on the secondary mirror.


PostPosted: Wed Feb 25, 2015 5:01 pm    Post subject: Reply with quote
I only have the ND filter.
As far as i recall i used it without, you're right about the filter. I just checked on this site: http://www.minoltaflashback.com/rf-lens.html They say the lens cannot be focussed correctly without a rear element, be it a normal filter or any other.

But i dont think it matters that much though, at least not on digital, look at this shot:

Red (RF Rokkor 250) by René Maly, on Flickr

To be absolutely sure i would have to test it again, without any element in it. The cat pic in this topic was shot with the ND filter.


PostPosted: Fri Mar 06, 2015 10:35 pm    Post subject: Reply with quote
I had a Makinon 300 mm f/5.6 Reflex Mirror for MC/MD with 67 mm filter thread and it was the worst lens I've seen so far. Sharpness was incredibly low and focus peaking on the NEX never showed anything.



Using a RF Rokkor 250 mm, now. Worlds apart! Sharpness is close to a Tele Rokkor 200 f/4, low distortion and zero CAs (after all, it's a mirror lens). Actual T-stop isn't that bad, either: Should be around T7.0 according to my calculations. But as it has been said before: hard to focus because of the shallow DOF.

For comparison:
Test of the RF 250 mm f/5.6
Test of the MD 200 mm f/4


PostPosted: Sat Mar 07, 2015 7:13 am    Post subject: Reply with quote
VLR: nice website!


PostPosted: Mon Jun 22, 2015 2:59 pm    Post subject: Reply with quote
I just tested my Minolta 250/5.6

Basic adjustments made in Aperture. Shot handheld with Sony NEX-7 in RAW. ISO 400 and 1/640.

No donuts when used properly or lots of donuts if you like that sort of thing. Incredible reach and sharpness for a lens so small. No CA.



100% crop
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