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Understanding medium format lenses


New Member
I would like to request help understanding two things about medium format lenses.

1. When a medium format lense is 50mm lets say, is that the same focal length as a 35mm format 50mm SLR lens?

2. I've noticed on medium format lens MTF charts that some of the ratings at 40 LP are much lower than 35mm SLR lenses. Should I be reading MTF charts for medium format lenses differently than I would for the 35mm SLR format?

1. Yes, it is he same focal length but represents a wide angle lens when using MF film.

2. The MF lenses can get by with lower resolution since the magnification to paper is much less. A MF lens is no doubt harder to design since it has to project a good image without distortion onto an area much larger than 24mmx36mm.
Rick, with respect ot #2, there are adapters to put medium format lenses on 35mm cameras. As the medium format lenses have reduced resolving pwer, the pictures are typically poor!
If you are talking about 6x6, then the equivalent 35mm focal length is approximately 0.6x.

Thus, a 50 is approx. 30mm, 80mm is approx. 50mm etc.

I suspect that the lower MTF curves away from the centre of the lens with MF is because the lenses are bigger, and some aberrations (eg chromatic and spherical) get worse with distance from the optical axis and thus increase the compromises for designers.
Nick, I was talking about 6X6. So if I multiply 0.6x the stated focal length, I'll be pretty close to a 35mm equivalent.

Do you happen to know what factor I should multiply for with 4.5X6, 6X7 and 6X8?

In addtion, I'd like to ask about enlargement sizes. How big can a 6X6 and 4.5X6 print go before it loses considerable quality?

Thanks to all who have contributed answers thus far.

Hi Rick,
For what its worth, i just finished a museum show of duratrans on ltbxes from chromes taken in mid 80's with 80mm hassy lens of world trade center hit by lightning during a storm downtown from my (then) loft window... anyway, i found that "acceptable" rez held at 48" x 48"., eg, you could see details inside the windows of the bldgs. i tried to go larger than this, (72"x72") but i found them less good. however i am still working with lab on this issue. more later. hope it helps. chromes were 100 asa as i remember. gotta love zeiss lenses! ( i am looking for a 100mm now. any help on this?)

hey, as a ps:
I shot lots of 35mm stuff with my hassy nikon adapter mount , and found not problem with rez or softness. au contraire, it was better than the nikon 50mm lens! but it was long focal and tight at f16 or 22 so... thats worth 2 cents.

As an addition to Stewart's post, there is that shows 'equivalent' focal lengths. To get the multiplication factor, you will have to do the maths yourself!

The question of aspect ratios adds complication. That's why I said approx.!

I see that you have also posted this on and there might be something in the archives as well.

On the print sizes, other factors like the film, how steady you are (there is nothing like enlargement to reveal softness from camera movement) and viewing distance.

The last is important as big pictures look sharper as you move back to an optimal spot, then get obscure with distance. Minutely examining large prints for sharpness is 'grainspotting'...the photographic equivalent of trainspotting (small joke)!