490mm f/6.2 OR-451M: fine OOF background for a $100

**ridax** · 13-Jun-2021, 11:04

The 490mm f/6.2 OR-451M (spelled OP-451M in Cyrillic) lens comes from an obsolete copy machine that was in use back in the USSR. Now these lenses find no use anywhere and are dirt cheap. They go for no more then $100 on e-bay and can be got as cheap as $20 locally, in the ex-USSR countries.

The OR-451M has 6 separate elements. It's perfectly symmetrical and is optimized for 1:1 reproduction work. At 2.2 kg (~4.84 lbs) it's not too heavy for its focal length and speed but rather bulky, 5" in diameter (just fine on a 6x6" lens board but hardly mountable on anything smaller). The barrel has no aperture scale as the aperture was originally operated through the copy machine itself, from outside of the lens. So LF users have to calibrate their aperture scales themselves.

http://lens-club.ru/public/files/opt...0e9765def8.gif
http://www.photohistory.ru/Pictures-1/Lens-OR-451-f.jpg
http://lens-club.ru/public/files/img...a5c2e0fe16.jpg

I bought the OR-451M out of pure curiosity and tested it and was really surprised by the out of focus background rendition of this purely technical copy lens.... In the center it was no worse than the background blur of my 480mm Dagor, with beautiful OOF at all apertures including wide open (while Dagors make great defocused backgrounds only at f/10 and smaller f-stops). And that's at no more than a 1/10 of the Dagor's price.

Well, no it is not equal to a Dagor in any other respects. It's a narrow angle lens covering just about 42° at small f-stops (15"/38cm field diagonal at infinity), and illuminates about 50° (18"/45cm at infinity) with the unsharp edges included.

It also shows a lot of coma that spoils both sharpness and out of focus rendition off center so I'd better stop it down well when using the OR-451M on 8x10". (At 1:1 coma is absent in any symmetrical lens so no special correction means are needed. But the closer to infinity the subject is, the more prominent coma becomes in a lens like this.) But on 4x5" the glass is fine at all apertures. It's somewhat on the soft side from wide open to f/10; the softness is gone at f/11. No I don't mean it's really soft focus; I compared the OR-451M to my 300mm f/4.5 Ross Express at wide apertures on 4x5" and found their pictures to look very similar. Though I actually liked the OR-451M more.

For a 490mm f/6.2, it's CHEAP. And at least it's quite a lot of fun. So I'd like to welcome everybody to buy and try the beasts - before they are all gone to the trash.... as nowadays nobody is interested in them.

P.S.: There also were the OR-451 (without the "M" in the name) that most probably was exactly the same glass in a somewhat different barrel and the OR-452 of the same type but of a different focal length that should probably also be a similar performer. But sorry I have no personal experience with those.

**ridax** · 24-Jun-2021, 10:56

I've also tried putting negative supplementary lenses in front of the OR-451M. A -0,25 diopter lens made too little difference to the lens's focal length, and a -0,75D spoiled both the sharpness and the out of focus rendition too much so I did not see any reason to use them. But the -0,5D made the lens to be about 640mm in focal length (the exact focal length of such a system depends on the distance between the supplementary lens and the main lens's entrance pupil, and I did not bother to find out where the OR-451M's entrance pupil was). Stopped down to f/32 or smaller, that 640mm combination is not bad in sharpness on 8x10" and still makes a great out of focus background.

The combination's f/32 means about the same iris opening as the f/22 for the OR-451M alone, without any supplementary lenses - as not only the focal length gets longer but also the entrance pupil of the system becomes smaller when an additional negative lens element is put at the front. If the supplementary lens is placed at the back of the main lens barrel the entrance pupil does not change, and besides the system needs far less bellows draw. But the image quality is much worse than with the supplementary lens at the front so the back placement is not recommended.

My supplementary lenses come from a common optical shop that sells consumer spectacles. The lenses I choose are manufactured decades ago when the modern standards requiring all spectacles to be UV-blocking were not in force yet. Fortunately, the local shop still has those old ones in stock, and as the modern lenses are certainly better for human eyes, the UV-transparent old ones are really cheap and still get no demand. (Being fond of blue-sensitive films, I prefer my camera lenses to pass as much violet and ultraviolet as possible to make my exposures shorter.) Those old spectacle lenses are glass, not plastic; though they are not coated (perhaps I'd have my -0,5D coated. The custom-made coating would probably cost more then the OR-451M itself but I also want a -0,5D for my 480mm Dagor so I guess its worth it).

The supplementary lenses I have are only 68mm in diameter. That seems way too small for the OR-451M that has much larger outer elements but as the resulting 640mm combination is a narrow angle one and also has to be stopped down quite a bit, the outer zones of the OR-451M's diameter are not in use. I made a low technology cardboard disc to fit into the front of the OR-451M barrel and mounted my supplementary lens into a 68mm hole in the center of the cardboard. No precision at all but it works. Dirt cheap, pretty fast to arrange, no trouble.

All the above means a fast 490mm + an f/32 640mm for the money somewhat less then the price of a pack of 8x10" FP4+ (1/4 of it in this part of the world actually). And my supplementary lens weighs 27 grams along with its cardboard mount - no problem carrying that additional 640mm along, whether it's used or not.