Which Focal length Lens for 5x7 Headshot ?

[Ron Bose]

Okay, so the other weekend I was doing successful half-shots with a 305 G-Claron with my relatively new Canham 5x7.

But, when I put on the 450mm Fujinon I didn't have the bellows to focus.

What kind of focal length lens will I need to get a reasonable headshot without being in the next room to focus ?

I've never had this issue on 4x5 !!

Any suggestions appreciated ...

Cheers,
Ron

[cotdt]

450mm sounds about right. get a bigger bellow.

[Ron Marshall]

450mm is perfect for what you want on 5x7. An extension lensboard is another option.

[BradS]

Hmmm, well the question is somewhat ill posed. Which 5x7 Canham do you have? The specs for the wood field say it has a maximum bellows extension of 660mm...

My shoulders are about 20 inches wide so, I figure you're looking at a reproduction ratio of something like 1/4 (image size / object size = 20 /5 ).

so, if we let u denote the image distance and V the object distance we can write..

u/v = 1/4 ==> 1/v = 1/4u

further, we know that if the thing at distance v is in focus, then...

1/f = 1/u + 1/v
(where f is the focal length of the lens)

combining these two we get...

1/f = 1/u + 1/4u = 4/4u + 1/4u = 5/4u

or,
f = 4U / 5

in words, the focal length is about 80 percent of the distance from lens to film plane.


With that in mind, if you have the Wood 5x7 Canham and 660mm of bellows....

f = 4 (660) / 5 = 528mm

and if you have the MQC with its shorter, 609mm maximum bellows,

f = 4 (609) / 5 = 487 mm

in either case, I guess you're about two meters away from your subject.

So, it would seem that the 450mm lens would work fine with either model Canham 5x7...???

Perhaps, your subject's shoulders are not so wide as mine?

[lenser]

GEEZ!!!!!! I couldn't figure out that kind of math when I was acing everything but trig in college.

I'll contribute the old fashioned way by looking into my lens kit. I've got a 10" Wallensak Veritar which I find perfect for head shots on 4x5 but which is listed in it's literature as being suitable for 5x7 in coverage and for individual portraiture.

Based on that, I would suggest that anything of 10 inches or longer will work, with the longer lenses providing for less distortion in really tight shots.

As others have observed, your bellows draw is your big limiting factor, so you'll probably do well in the 250 to 360mm range.

Have fun.

Tim

[Ron Bose]

Thanks guys, especially BradS, let go do the math and I'll give it another try.

I have the KBC which is the Woodie.

[Steven Barall]

Math??? We don't need no stinking math!!! It sounds to me that the people you're photographing are just to darn small. Hang around a MacDonald's parking lot and get yourself some larger subjects. I just love an elegant solution to a difficult problem.

[Ken Lee]

As a rule of thumb, 5x7 lenses are 1/3 longer than 4x5.

Again in approximate values, the diagnonal of a 4x5 sheet is around 160mm, and the diagonal of a 5x7 sheet is around 215mm. Those numbers are commonly be used to define a "normal" lens length for each format.

Given that a "portrait" lens is somewhere around 1.5 - 2x the length of a "normal" lens, this suggests that on 5x7, the equivalent length is somewhere between 325 and 430 - depending on how far away you like to place the camera from the subject.

I use 300mm and 360mm lenses for this purpose on 5x7.

Keep in mind that depth of field starts to get rather narrow at these lengths, even when the lens is stopped down.

[Sal Santamaura]

Whole lotta unnecessary complication. In large format, a good rule of thumb for portraits is to simply add up the film's dimensions. 5+7=12. 12" = 300mm. Good to go.

[Leonard Evens]

As BradS pointed out, it all depends on your definition of "head shot". Let me redo the calculation and reduce it to essentials. The relevant relationship is

bellows extension divided by focal length = magnification plus 1

(Magnification is something of a misnomer. It is actually the reduction ratio going from subject to image.)

Using the usual symbols, this is

v/f = M + 1

My shoulders are about 20 inches across, so for a 5 inches wide frame, the reproduction ratio is 5:20 = 1:4 for a magnification of 1/4. So

v/f = 1/4 + 1 = 5/4.

That says that the bellows extension will be 1/4 larger than the focal length. If f = 450, this means you need a bellows extension of about 563 mm. If you can extend the bellows that far, you should be able to do it. The corresponding distance from the lens to the subject would be 4 times that, i.e., 2.25 meters or a little under 7 1/2 feet.

The distance of the subject from the camera is very important since it will affect the appearance of the subject in important ways. Generally, if the subject is closer to the camera, features like the nose may appear exaggerated in the print. It is generally assumed that such effects will be minimized if the subject is at least 6 feet from the camera, and 8 feet would be even better. The larger the format, the easier this is to do without using excessively long lenses relative to the "normal" format. This is fortunate because such lenses become increasingly more difficult to obtain as the format size increases.

If you can't place the subject that far from the camera, you will have to use a shorter lens to fill the frame without overflowing it in the short dimension. If you can place the subject that far and you can't focus with a 450 mm lens, either your maximum bellows extension is unusually short or there is something screwy going on. BradS suggests that the bellows extension is adequate.

Apparently your quandary is that you have a very limited amount of space. If a 305 mm lens worked, filling the frame, we can use the following formula to estimate the distance of the subject to the camera.

u/f = 1/M +1

In your case, we foound M needs to be about 1/4, so this yields u/f = 5. That means the subject disance was 5 times to focal length, which is 1525 mm = 1.535 meters or about 5 feet, which is perhaps a trifle short from the oint of view of facial "distortion". Note that if you know the magnification (which in this case is determined by the desire to fill the frame with "head and shoulders") and the subject distance, there is only one focal length that will do it. The bellows extension as before is determined by the formula

v/f = M + 1

which as before is 5/4. If you increase the focal length, you have to increase v proportionately to compensate and maintain the ratio at 5/4. Eventually, you will run out of bellows extension. If your maximum bellows extension is about 600 mm, then the longest focal length you can use is about 4/5 of that or 480 mm. But there is no real point in choosing a longer focal length lens, which might make it more difficult to focus.

My suggestion is to find a room which allows you to put the subject further from the lens. As we saw above, this has little to do with you maximum bellows extension.