Fstop group shots and different lenses

[AndreG]

Today I'm trying to work out f stops with groups shots and different lenses and trying to work out this depth of field calculator but struggling so wondered if anyone could help with some real world examples.

I was talking to a pro who was not feeling very helpful and just told me that F9 would be a good starting point for a group shot, so I'm looking at the depth of field calculators and they have a range of 'acceptable sharpness' for focus - what does that mean, I've tried recreating a shot from the depth of field calculator with some sticks in a line and I'd say that they were not acceptably sharp at the distances that the calculator says they are - am I just being picky?!

In real world terms (going on the variables on the depth of field calculator) - if I had my D90 with my 28-70 lens on it taking a photo of a group of 20 people stood 3 deep, and was stood far enough back to fit them all in, what F stop would I want to use to get front and back rows in focus and where would I aim the focus point?

If I then swapped to a bigger lens like a 200 and moved back so it's the same picture, would the f-stop needed change?

Many thanks and so sorry for the dumb questions

 

[Phil V]

This is where questions and book learning lead people into blind alleys.
If I then swapped to a bigger lens like a 200 and moved back so it's the same picture, would the f-stop needed change?

You'd need to be a football field away to get them in. Do you really need to know the answer? How much use is that knowledge.

To be fair, your 'not very helpful' pro gave you as good an answer as you'll get. I never go beyond f11 for large groups at weddings (could be 100 + people) because I shoot them in a way that flattens the distance front to back - using a wide angle and shooting from above.

I've no idea whether you're just being picky, because you haven't given the details of what you were trying. But the focus point should be about 1/3 of the way into the group, because DoF stretches further back than it does forward. The sticks experiment should have shown you that, When it 'failed' did you close the aperture another stop and see the difference?

The thing about 'acceptably sharp' is that it's subjective by nature, and prone to so many variables books have been written on the subject. You only need to learn what works for you. I'm guessing a crop sensor, so at 28mm you're not really getting a wide angle, is there something wider you could try?

 

[HoppyUK]

Depth of field is an illusion, in that there can only ever be one plane of sharpest focus. However, all DoF calcs are based on the international standard that says the human eye cannot detect details smaller than 0.2mm in a print 10in wide when viewed at a distance equal to the diagonal, ie about 12in. Therefore, anything smaller than that will appear perfectly sharp and is said to fall within the zone of depth of field. This standard holds good for any size of image so long as the viewing distance (the diagonal length) remains the same. If you start viewing at 100% on screen and view closely, all these calcs are void.

This is the standard used in all decent DoF calculators, like this one http://www.dofmaster.com/dofjs.html and bear in mind that distance is measured from the front of the lens, not the sensor - can make a difference with close ups. Use DoFmaster calcs to establish a baseline and do some tests. If you find that at the margins of DoF things are not acceptably sharp to your eye, then just use a stop higher, ie if the calculator says f/5.6 should be okay, then use f/8.

Couple of other things: if the subject is framed the same, then DoF will be the same, at the same f/number, regardless of the lens focal length. If you're shooting groups with a wide-angle, keep people away from the sides of the frame or they'll get pulled out of shape and look fat

 

[DemiLion]

I shot a group of 32 this afternoon ranked three deep (well three and a half actually) at f/8, 1/160th 35mm. I'd say that the advice you were given was actually not bad.

 

[EOS_JD]

Focal length itself actually does not affect the depth of field. There's only 2 things that affect the depth of field. That is the aperture and magnification of the subject.

Magnification being how large the subject appears in the image and is a mix of the distance to subject and focal length used.

 

[DemiLion]

So that's three things then?

Most people understand it as a triangle between aperture, subject distance and focal length.

 

[EOS_JD]

So that's three things then?

Most people understand it as a triangle between aperture, subject distance and focal length.

No 2 because its only magnification
I argued your point on here with a guy a few years ago and he proved there was a difference but I can't remember it now lol.