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swmcl
11-Feb-2012, 14:27
Hello to all,

I'm doing my sums ... with the aid of Bob Atkins interactive calculator that I've loaded onto the computer.

So I'll put some figures down and I ask if I'm doing things wrong.

I start with a Heliar 300mm f4.5 lens and I take my portrait at 3m distance with an 8x10 camera. The CoC is 300 microns, the magnification is 0.1111 and the DOF is 24.35cm all according to the calculator.

Now. I go the Xenotar 150 on 4x5.

The CoC is 150 microns, the aperture is f2.8 the magnification is 0.05555. I get a DOF of 24.35cm and a distance o 2.854m.

If you can appreciate it, I'm trying to replicate the image on 8x10 with the 4x5 camera. Exactly the same image means half the magnification right ?? But why only the slight change in distance from 3m to 2.854m ? Is this figure correct ?

Also, it looks by the calcs that the Heliar is getting a shallower DOF than the Xenotar for the same image. Is that right ?

I also have another lens of 205mm FL that has a max aperture of f3.5. If I shoot that on 5x7 then the following figures occur. The CoC is 200 microns, the magnification is 0.77777 the distance is 2.843m and the DOF is 25.04cm.

In summary:

The Heliar 300mm f4.5 on 8x10 has the shallowest DOF of all these lenses and beats the Xenotar 150mm on 4x5 easily. The 205mm f3.5 lens on 5x7 also beats the Xenotar 150mm on 4x5. All assuming the exact same portrait image.

I still think something is wrong. Please enlighten me !

Rgds.

eddie
11-Feb-2012, 15:08
use the same lens for both.

make the subject the same size on each negative. (this means you will need to be farther away with the 4x5 camera). viola! you have replicated it.

no, you can not do it the way you are thinking.

eddie

rdenney
11-Feb-2012, 23:07
Depth of field is a function of magnification, f-stop, and what you declare to be "in apparent focus" (i.e., the circle of confusion). When you reduce magnification with a shorter lens, DOF increases. Changing the format has no effect, except that it might tempt you to enlarge the picture more to achieve the same print size, in which case the magnification is increased back to where it was. The difference that remains is that the aperture was a different ratio of the shorter focal length, and enlarging more doesn't compensate for that. So, you still end up with more depth of field with the shorter lens, even when the prints are the same size.

The equation for the hyperfocal distance (the nearest distance you can focus and still include infinity within the depth of field), is H = f^2/(f-ratio X circle of confusion) + f, where f is the focal length, and f-ratio is the number you see on the aperture scale. So, the hyperfocal distance increases as the square of focal length, while magnfication increases linearly.

Hyperfocal distance is the easiest single number to describe depth of field. If two scenarios have the same hyperfocal distance, they have the same depth of field at any focus distance. A larger hyperfocal distance means less depth of field.

So, if you double the focal length (150 to 300), the hyperfocal distance will increase by the square of 2, all else equal. If you cut the circle of confusion in half to account for the fact that the larger negative needs half the enlargement to make the same size print, the hyperfocal distance increases by half the square of the focal length. So, going from 4x5 to 8x10 and doubling the focal length increases the hyperfocal distance by 2 (read: less DOF), if you reduce the circle of confusion by half (i.e., if you are judging based on the same print size). Doubling the focal length without changing the format or enlargement increases it by 4. Either way, it increases.

To compensate, the f-number has to double.

Taking your example 8x10 case, 300mm at f/4.5, circle of confusion of 0.3mm. The hyperfocal distance is (300 x 300) / (4.5 x 0.3) + 300 = 67 meters.

Equivalent 4x5 case: 150mm at f/2.25 (half of 4.5), same print size and therefore a circle of confusion of 0.15mm (because it will be enlarged twice as much). The hyperfocal distance is (150 x 150) / (2.25 x 0.15) + 150 = 67 meters. (Actually, it's 150 mm less, but that is not significant until you get down into the macro range.) You know that it provides the same depth of field at f/2.25 because it has the same hyperfocal distance.

Given that the Xenotar opens no wider than f/2.8, your observation is correct--it will necessarily provide more depth of field wide open on 4x5 than the Heliar will on 8x10, given the same subject framing and print size.

Rick "noting that all the subsequent formulas for near and far limits of DOF are derived from the hyperfocal distance" Denney

Ken Lee
12-Feb-2012, 01:00
In a nutshell, when you double the focal length, you need 2 stops smaller aperture to get the same depth of field.

For example, a 75mm lens @ f/8 has the same depth of field as a 150mm lens @ f/16, which has the same depth of field as a 300mm lens @ f/32 and a 600mm lens @ f/64.

Small-format shooters who like strong background blur in portraits, are obliged to use very wide apertures. Large format shooters who like strong depth of field, are obliged to use small apertures.

swmcl
12-Feb-2012, 01:07
Mr Denney,

You're a champ !

I did go back and try again though with the same calculator ...

What I see is that indeed the Xenotar has the thinnest DoF on 4x5. However, as the size of the photo is increased other lenses can achieve the same or less DoF because the Xenotar has a limited image circle.

The Xenar 178mm f3.5 for example, beats the Xenotar with a shallower DoF for the same image on 5x7. The Xenotar in my image size had 28.88cm DoF on 4x5 where the Xenar 178mm has a DoF of 25.04 on 5x7. The Heliar 300mm f4.5 on 8x10 however, beats both with a DoF of 24.4cm at the same magnification.

My conclusion is that the silly prices for Xenotars are not justified. The Xenar on a 5x7 is shallower and not much larger to tote around. The Heliar on an 8x10 is a much bigger beastie though.

:-)

Hermes07
12-Feb-2012, 02:00
Mr Denney,

You're a champ !

I did go back and try again though with the same calculator ...

What I see is that indeed the Xenotar has the thinnest DoF on 4x5. However, as the size of the photo is increased other lenses can achieve the same or less DoF because the Xenotar has a limited image circle.

The Xenar 178mm f3.5 for example, beats the Xenotar with a shallower DoF for the same image on 5x7. The Xenotar in my image size had 28.88cm DoF on 4x5 where the Xenar 178mm has a DoF of 25.04 on 5x7. The Heliar 300mm f4.5 on 8x10 however, beats both with a DoF of 24.4cm at the same magnification.

My conclusion is that the silly prices for Xenotars are not justified. The Xenar on a 5x7 is shallower and not much larger to tote around. The Heliar on an 8x10 is a much bigger beastie though.

:-)

It's not just about depth of field when it comes to prices though.. the Xenotar is still ultimately faster which means a shorter exposure time if you're using an alternative process and are desperate for as much light as possible.

If shallow depth of field is your only concern, you'll find it tricky to match on 4x5, what even modest lenses can do on larger formats. A (commonplace and fairly cheap) 30 inch process lens @ f/9 shooting 16x20s will need a 7.5 inch f/2.2 lens on 4x5.

ic-racer
12-Feb-2012, 08:37
To replicate things with the 4x5 you need a subject with a head that is 1/2 the size. (Seriously!).

genotypewriter
13-Feb-2012, 05:56
If shallow depth of field is your only concern, you'll find it tricky to match on 4x5, what even modest lenses can do on larger formats. A (commonplace and fairly cheap) 30 inch process lens @ f/9 shooting 16x20s will need a 7.5 inch f/2.2 lens on 4x5.

Hmm how cheap is 16x20, say, if I want an E6 sheet? :)