Pentax 67 DOF

[douglas_j._moo1]

I am investigating med. format for landscapes, but am worried about

relative lack of DOF for extreme near-far compositions. I know CofC

can be bigger with this format, etc., but would still like more

specifics. Could anyone who uses a 6x7 med. format camera (without

tilts) + a 55 mm. lens tell me exactly how close one can keep

something in focus at f22 with infin. also in focus? Thanks.

[djphoto]

The depth of field of any lens of a given focal length is exactly the same at any given aperture as any other lens of the same focal length, no matter what camera it is made for. The depth of field for a 55mm Pentax 67 lens at any given aperture is exactly the same as for a 55mm lens on a 35mm camera at the same aperture. If you have a manual lens of about 50-or-so-mm, you can read your approximate near distance from the depth-of-field scale by setting f22 at the infinity mark.

[douglas_j._moo1]

This is original poster. I understand that DOF is focal length dependent. But I also know that lens barrel markings are often very optomistic and published DOF tables vary a lot. I would still like to hear from people actually using the set up (55 mm. lens at f22) what kind of near DOF they can achieve with acceptable sharpness throughout (say, in a 16x20 enlargement). I can tell you exactly what this would be for me in 35mm; but I still wonder whether the larger starting negative in 6x7 would make a difference in the "real world."

[gene_crumpler6]

Douglas:

 

I use the 55mm f4 for landscape work. I don't have the lens with me, but the rule of thumb for 16x20 prints(which is what I use the 6x7 for)is to use the DOF markings for 2 stops less than the actual apreture in use. I've found that with the lens at f22, I can get about 8-10 feet to infinity reasonably sharp. The f4 is the only lens I've ever tested that gives the same lp/mm from f4 to f22, rather unusual! The pentax 55 F4(newest version) is one of the best lenses you can purchase in MF photography. It is the lens that stays on my camera 80-90% of the time. It approaches the sharpness (lp/mm against lp/mm) of my best nikkors).

[Bill C]

Douglas,

 

I have enjoyed reading through the forum for some months now, and thought maybe I ought to contribute a little! Although you specifically asked for responses from users of 6x7 cameras (I�m not), I hope I can help out. You seem to have somewhat discounted Mr. Jenkins� response; however, I believe he is exactly right and that if you carry his line of thinking a little further, you will find that you already know what you are trying to find out.

 

You indicated that you already know what the results would be for 35mm film using a 55mm lens at f/22. All else being equal, this should be exactly equivalent to using only the center portion (same size as 35mm film) of a Pentax 6x7 negative shot with a 55mm f/22 lens. If you want to evaluate a 16x20 from the Pentax 6x7, make an 8x10 from your 35mm film (shot w/55 mm lens at f/22). Then, pretend the 8x10 was cut out from the center area of a 16x20! This method IS LEGITIMATE; it assumes the 16x20 would be made from a nominal 2" x 2.5" section of negative and that the 8x10 is made from a nominal 1" x 1.25" section of negative.

 

If you rely on the opinions of others as to "acceptable sharpness", it�s hard to know how critical they are being without actually seeing their work. Thus, your own tests on the specific film and subject matter you are looking at are probably the best source of information. Happy Shooting !

[gene_crumpler6]

The above suggestion is a good one within certain limits. One, each manufacturer has its own criteria for acceptable DOF. So the DOF markers on a 35mm lens do not correlate with the DOF markers on a MF lens. The MF manufacturer assumes that the bigger negative will be enlarged less. My rule of thumb for my 35mm nikkor lenses is use the DOF markers for one larger stop. For the 67 I use 2 stops.

 

Second, another criteria for "acceptable" sharpness is the viewing distance from the print. A I recall the "correct" viewing distance for a print is the diagonal of the print. Thus for an 8x10, it is about 12 inches and for a 16x20 is is about 25 inches. Using the above example, you would have to hold the 8x10 at a distance of 25 inches.

Hard to do for us complusive grain sniffers! I use a 7x loupe to evaluate my prints! If the image edges under the 7x loupe are not distinct, then it is not really sharp!

 

As usual on this forum, the ultimate advice is to rent or borrow the camera and lens and try it your self!

[tom_goodrick]

I have been pleasantly surprized to see good results in near-far pictures with my 45 f4 judging by examining some 4x5 prints with a 5x loupe. I have used the DOF equations referenced elsewhere on this net to study the problem. The numbers are scary. You certainly must consider DOF in almost all shots. But, each Pentax lens has a good stop-down lever that you can hold or set while examining the scene carefully. This has given me confidence that my near-far shots will work. Clearly, however, the 6x7 is a compromise of convenience over the 4x5 which will obviously do the job.

[gene_crumpler6]

A follow up on my earlier response. The DOF markers at f11 (which is what I use for the f22 setting on my 55mm) check out at about 9 1/2 feet to infinity. This will give good DOF in a 16x20.