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View Full Version : 8x10 150mm = 120mm by cropped how much?

Richard Fenner
26-Sep-2004, 13:37
Does anyone roughly know (or can work out!) what size film sheet would be left, if on 8x10, using a 120mm lens, the image area was cropped down to roughly equal the image a 150mm lens would create?

Ernest Purdum
26-Sep-2004, 13:45
If I understand the question, ir can't be answered as posed, because lenses vary widely in coverage. If you can be specific as to what lenses are concerned, perhaps someone wil be able to answer.

Richard Fenner
26-Sep-2004, 14:49
OK, I thought this was easy but I guess it isn't!

Nikkor SW lenses - 120mm and 150mm, probably at f32 for each.

Glenn Kroeger
26-Sep-2004, 15:19
Lets say the 150mm images an object at 8" long. The 120 will image it at 120/150 as big, or 4/5 (80%) so the 8" object is now 6.4" long. So the 8x10" image area become 6.4x8" with the shorter lens. Proportional to focal length!

Jason Greenberg Motamedi
26-Sep-2004, 15:21
If I understand correctly, you are asking for equivalent angles of view: If I have done my math correctly (which is doubtful), I think the answer should be 6" x 8", more or less a full plate...

150mm / 305mm (diagonal for 8x10) = 120mm / x (diagonal for new size)

so the new size should have a diagonal of 120/.49 or 245mm.. 6x8 works just fine for this

Ernest Purdum
26-Sep-2004, 15:25
I think I've got it, but I'm notoriously weak in math, so anybody jump in to correct me if I am wrong. (I think I do have a better idea of the question, now, too.) 120 is to 150 as X is to whatever value we choose should work. If we use 6 as an image height, I get 4.8. (120 X 6 = 720. 720 divided by 150 = 4.8.) To get area, we'd have to work it for image width as well. Solving it for a width of 8, I get 120 8 = 960. 960 divided by 150 = 6.4.

Our original area was 48 square inches. With the 120mm lens we are now using 30.72 square inches. somewhat less than half of the nominal 80 squsre inches of the 8 X 10 negative.

Richard Fenner
26-Sep-2004, 15:39
Glen and Jason, although your answers are slightly different, they're roughly what I expected. Ernest - it's clear I needed to better phrase the question, but even so, I'm bamboozled by your answer!

Whereand why does the 'original area of 48 square inches' turn into 30.7 square inches? I can see original area of 80 square inches turning into 48 square inches, but not the next step in your calculations!

I've often felt that people provide inadequate information to let others really understand what they're doing and why, and limiting they type of answer they can provide isn't always useful. So, here's what I'm doing:
I want to shoot some ceilings previously shot on 6x7 with a 50mm lens. I like the angle it provides, and haven't gone wider than 150mm on 8x10 for this reason. I want to reshoot this on as large film as I can. Cambo Wide would let me go to 75mm for 4x5, although they're pretty uncommon secondhand, and the new Wide DS is pretty expensive for a camera with movements I don't need. So, looking a bit larger at 8x10, the Hobo is the best way of doing it. But the Hobo only goes up to 120mm, so I need to find out if I can create the angle of view I want with film sufficiently larger than 4x5 to make it worth using.

There is possibly an alternative way of doing this, but I haven't tried it yet. I've tried focussing with a camera on a tripod pointed directly up, and it can be done, but with the 150mm on 8x10 you're still a couple of feet higher than you'd like to be. It may be that I could learn how to set the focus (anywhere between a ceiling of 10ft and 30ft) without focussing on the gg, which would let me have the camera much closer to the ground. Is this easily done? I've never had rails in the image area with the 150mm pointed normally.

Leonard Evens
26-Sep-2004, 16:03
According to www.largeformatphotography.info/lenseslist.html, the image circle, probably at f/22, is 312 mm, barely enought to cover the 8 x 10 format. Similarly, the Nikkor 150 has an image circle of 400 mm, which leaves some room for movement.

It is not entirely clear what you mean by cropping down. If you want to crop the image from the 120 mm lens, so the resulting image presents the same angle of view as a 150 mm lens, then you need to crop by 120/150 = 4/5 or 80 percent. The image circles would not be relevant. As long as you have a complete image you are satisfied with from the 120 mm lens, you can choose any section of it with dimensions 80 percent of the original image on 8 x 10 film, and 20 percent of the 120 mm image would be left in each direction. If you want to mask the 8 x 10 film so that the angle of view is that of a 150 mm lens, just make the dimensions of the opening 80 percent of those for the 8 x 10 frame size. That would be a bit less than 6.4 x 8 inches because the 8 x 10 frame is a bit less than 8 x 10 inches to start with. I don't do 8 x 10, so I don't know the exact size, but that information should be readily available. What you cover up with the mask would be film that is left over, and it would presumably be blank if your mask worked well. Or perhaps you want to cut down the film to a smaller size and somehow mount it in a film holder. Well, you can also do that and the dimensions would be the same.

If you mean something else, let us know.

Glenn Kroeger
26-Sep-2004, 16:57
My answer agrees with Leonard. Image circles are irrelevant to angle of view calculations (assuming they are large enough to cover).

With a 120 on the Hobo with 8x10 film, you will be able to crop down to approximately 6.4"x9" to replicate your angle of view.

If you want to get more exact, you need to know the actual image dimension of your 6x7. Different 6x7 cameras and backs make slightly different image sizes, usually about 56mm x 67mm.

Remember, with sheet films, both 4x5 and 8x10, you need to allow some room on the edges since actual image area isn't that big, and processing clips often intrude into the image area. So you are much better off to go slightly wider and crop to your desired image area... on 4x5 you would be safer with a 65mm than 75mm lens for that reason.

Glenn Kroeger
26-Sep-2004, 17:00
Typo, that should have been 6.4x8"

Richard Fenner
27-Sep-2004, 01:48
Thanks people. Yes Leonard, this is what I meant but seemed unable to properly describe.

I was curious about the need to provide lens data - it seemed to me as long as it covered (and I knew they did), it was irrelevant. Still, in the past what people think to be irrelevant is sometimes relevant. I realise at f22 the 120mm only has an image circle of 312mm, but that should mean at f32 it covers, and this scenario requires no movement. Previous threads on this lens indicate no problems covering 8x10 with only a whisker of movement.

Glenn, I'm often puzzled when people complain about clip marks in the image area. There is a solution - change labs!

Ernest Purdum
27-Sep-2004, 06:55
Regarding the focusing problem, I suppose you don't have a means of measuring the ceiling height. If you did, you could place the camera at that distance from a wall, focus, lock the focus and set the camera on the floor lens up.

With an unknown ceiling height, a somewhat more elaborate procedure should work. With the camera on the tripod pointed up, focus on the ceiling and lock the focus. Measure the distance from the camera back to the floor. Now point the camera at a wall and move the camera until the wall is in focus (a dolly or cart of some sort would be very helpful). Now move the camera backward by the distnce of the back to floor measurement taken earlier. Refocus and lock.