Someone could explain...accessory for crop bodies

[magnet-2009]

Someone could explain...accessory for crop bodies solves the metering problems.

Details?



http://www.penta-club.ru/forum/index.php?showtopic=41859&st=0

[visualopsins]

google translation of that web page:

http://translate.google.com/translate?js=n&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.penta-club.ru%2Fforum%2Findex.php%3Fshowtopic%3D41859%26st%3D0&sl=ru&tl=en&history_state0=

looks like a simple mask over rear of lens to limit projected image circle size. masks full-frame image circle to cover only crop-sensor.

my idea was to place mask over front of lens, but each focal length needs a different size mask. the rear mask is same size opening for all focal length lenses, maybe easier.

idea is less light into metering -- only crop-size image circle not full frame size.

[aoleg]

Yep, the thread describes the issue of cropped Pentax bodies over-exposing with MF glass. The mask (or, rather, screen) put at the rear of some lenses improves exposure and contrast without affecting other characteristics such as light fall-off and effective light transmission of the lens, if implemented properly.

The thread has images to illustrate masks that were properly and inproperly done, and discusses the effect it has on bokeh if implemented in a deliberately wrong way. They figured, it's possible to obtain characteristic 'swirly' look when using the mask too small; this works similarly to large-aperture lenses manufactures in slim mounts (e.g. a 85/1.5 lens in M42).

Anyways, it's an easy way to get something new - and possibly better - out of old glass used on cropped sensors.

[Orio]

Simpler to adjust exposure compensation with + or - values.
I assume that Pentax cameras should have that feature.

[Nesster]

The results are interesting and looks like there's some experimentation possible.

In my experience the pentax digital bodies under expose by >1 stop with mf lenses. Foil or a metal wire round the lens to short the mount contacts usually improves the situation.

And yes, exposure comp.

[LucisPictor]

[Nesster]

Some lens mounts seem to have just such an aperture - I'm thinking I've seen it in Minolta, and maybe some others.

I'd agree though really am clueless - you still see the whole image, though possibly not that much darker - isn't one of the roles of the rear element the collection and direction of all those rays to the film? I.e. the aperture cuts light and edge aberrations by blocking their passage through the lens... by the time you're baffling the rear end, I would think most of the effect is on all those 'wasted' rays that are looking for reflective surfaces along the light path to reflect off of? Depending on where and what the meter sensors are reading, removing this extra light may make the reading more correct.

[Nesster]

Here's links to a couple of Adaptall-2 mounts - Minolta MD and a couple of Canon

http://i297.photobucket.com/albums/mm203/ideastoprint/Adaptall-2_MD_1.jpg

http://static.photo.net/attachments/bboard/00H/00HnIr-31949884.jpg

Some adaptalls have the baffle some do not. I never could figure why.

[Farside]

[Nesster]

the internet's a wonderful thing -- seems even some current lenses have these rectangular baffles - usual explanation seems to have to do with veiling flare etc.

Here's one of many Konica to Oly conversions where the style is to add a baffle:
http://blog.lewander.com/2007/07/konica-hexonon-lens-on-fourthirds-mount.html

[spiralcity]

[aoleg]