Infinity focus on 75mm Komura seems too long?

[William Akey]

I just purchased a 75mm Komura lens. It is in great shape and the shutter seems to function well. However, I have noted one peculiarity of this lens that has me rather puzzled. I set it up on a flat Technica style lens board for use on my 4x5 Tachihara. In determining the infinity for this lens I soon discovered that the infinity focus is about 3 mm longer than the infinity focus of my 90mm Fujinon! How can this be?

I rechecked the infinity focus of my 90mm lens and it is pretty much bang on where it should be in terms of the distance between the lens board and the ground glass. Is there something weird in the construction of the 75mm Komura that would make the infinity focus be so far away from the film plane?

Any thoughts on this would be greatly appreciated.

Thanks

William

[David A. Goldfarb]

It's probably a retrofocus lens. Retrofocus lenses tend not to be as sharp as traditional lenses, but they also usually have less falloff of illumination and they give you a little more bellows flexibility for movements.

[Kerry L. Thalmann]

Yep, David's correct. The Komura SuperWide lenses are retrofocus designs. As far as I know, they are the only retrofocus wide angles ever made for large format. On the other hand, all 35mm and medium format SLR wide angles are retrofocus designs (they have to be to clear the mirror in an SLR system). Retrofocus designs generally have more distortion than "simple" wide angles, but as you have discovered and David has mentioned, they do have a couple distinct advantages. First, they have a longer flange-to-focal distance than "simple" wide angles. I don't have the specs on the 75mm Komura, but I believe the ftf is somwhere around 100mm (give or take). That makes it easier to use on many cameras than a conventional 75mm. On many cameras, a bag bellows will be required to use a regular 75mm - or movements may be restricted if the camera does not have an interchangeable bellows. The second advantage is less illumination fall-off. Since the lens actually requires about the same extension as your 90mm Fujinon, fall-off should also be comparable. Many people (myself included) find a center filter necessary when shooting 4x5 color transparancy film with a 75mm lens. You'll most likely not need one with your 75mm Komura.

Kerry

[William Akey]

Thanks, David.

Ahh.. make sense now. I just read a brief reference to retrofocus lenses a few days ago. I didn't realize that I owned one of them until now :-). Well, I hope the sharpness will be acceptable -- certainly, it is marvellous having full movements on a 75mm lens.

Do you know if the nodal point of retrofocus lenses is set back further behind the shutter in much the same way that the nodal point is set further forward on a true telephoto lens? Do the retrofocus lenses suffer the same problems with image tilting that true telephoto lenses do?

William

[David A. Goldfarb]

Yes, that's exactly how a retrofocus lens works--just the opposite of a telephoto, and as such, tilts and swings will require a little rise/fall and shift to recompose in the same way..

[Dan Fromm]

Guys, are you sure the lens is retrofocus?

I ask because my 65/8 Acugon's flange-to-film distance is considerably larger than that of the 65/6.8 Raptar it replaced. The Acugon is in the same design family as the Super Angulon, and as far as I know the SA isn't retrofocus.

Please correct my misconceptions, if indeed I have 'em.

Thanks,

Dan

[Kerry L. Thalmann]

Dan,

I don't have data on the 65mm f8 Acugon, but the 65mm f8 Super Angulon has an ftf of 70.5mm. Most modern wide angles have an ftf slightly longer than their focal length. Keep in mind that these are very thick lenses (56mm total length in the case of the 65mm SA, which is small compared to many of the newer wide angles). Neither the principal focal point, nor the shutter flange necessarily lie in the exact physical center of the lens. That does not make them retrofocus.

The Super Wide Komura lenses, however, are true retrofocus designs. The 75mm has an ftf of ~100mm. This is 33% larger than the focal length and significantly more than any other 75mm large format wide angle. Other modern 75mm lenses typically have an ftf in the 78 - 85mm range.

Kerry

[Dan Fromm]

Thanks, Kerry.