What lens for full body isolation ?


Link Posted 25/06/2010 - 13:18
Does anyone have a firm grasp on the relationship between focal length, aperture, subject distance, subject size & depth of field

Basically I'm looking to do a full body portrait of a couple & whilst obviously they need to be in full focus, I'd like to blitz the background as much as possible.

I'm shooting full frame. I have fast 20, 28, 50 & 85mm lenses plus I also have a 150mm f2.8. - I understand each will have its own effect on perspective, but which will isolate the subjects the most ?
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Link Posted 25/06/2010 - 13:25
I really don't want to start a new discussion on lens equivalents, etc. So I suggest that you google dof or focal length, Wikipedia has some good articles on the subject.
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K20D, Optio I10, DA 18-55 1:3.5-5.6 AL II, A 1:1.7/50, D FA 1:2.8/100 Macro, Sigma 70-300 1:4-5.6 APO DG Macro, Pentax AF 360FGZ


Link Posted 25/06/2010 - 13:30
terje-l wrote:
I really don't want to start a new discussion on lens equivalents

To be fair Terry, it's RR that's starting a new discussion here. What's wrong with tapping into the Forum for some helpful advice?

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Link Posted 25/06/2010 - 13:31
Where are you taking the shots? This will have a big impact on the choice of lens - if your room is small, then you'll have trouble getting full length on 85 or 150.

The DOFMaster is helpful: http://www.dofmaster.com/dofjs.html
and this: http://www.tawbaware.com/maxlyons/calc.htm

20mm is going to be rather distorted (unless that's the 'look' they want!).
So, I'd say that unless you are shooting outdoors then the 50mm or 28mm are the ones to start with

Failing that, you can always go for post-processing. Use a tool like Topaz Remask to quickly isolate the subject and then replace the background (or use the Lens Blur feature of Photoshop).

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Last Edited by MattMatic on 25/06/2010 - 13:33


Link Posted 25/06/2010 - 13:36
Equivalents need not come into the discussion Terry. Whilst the wiki page on dof has some quite cracking equations I was wondering if anyone could give me the benefit of their experience.

I'm guessing it'll be between the 85mm f1.4 & 150mm f2.8 (although I'd like to close them down a stop for better performance) - if the wife wasn't in work, I'd get her to be my Guinea pig.
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Link Posted 25/06/2010 - 13:40
According to the calculators...

For 85mm on FF, you'll have to be about 5m away from the subject (to get 2m long side)

For 150mm on FF, you'll have to be about 9m away from the subject.

You'll need a big space

And if you're shooting fairly wide open then watch your angle on the couple (or their relative placements) as you may end up getting caught out with DoF.

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Last Edited by MattMatic on 25/06/2010 - 13:41


Link Posted 25/06/2010 - 13:43
Those calculators are handy Matt thanks. It's just working out how far away you'll need to be to get the couple in frame. I'll do some guesstimation in the back garden & work it from there.

I'll be taking the shots outdoors in quite a large area surrounded by mature trees, so space isn't an issue really.
I'd agree 20mm is a bit extreme, but I may whack it on if their patience is holding out.
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Link Posted 25/06/2010 - 13:49
Thanks again Matt, you're quicker than me on the draw today, that's really helpful.

It's basically for my Nephews wedding, I've been asked last minute & whilst I'd obviously like to say no for an easy life, they can't afford a pro tog & whilst having my reservations, I'm confident that my efforts will be far better than other relatives taking snaps with their compacts at least.

I will be careful about their relative position & adjust accordingly. I think I'll nag my wife into doing a dry run test tomorrow so that I can see what I can get away with. Perhaps focus, then ask her to take a step back to check if she's still within the dof.
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Daniel Bridge

Link Posted 25/06/2010 - 13:56
Try to find a viewpoint where you'll be able to put as much distance between the couple and the background. I know it sounds basic, but there may be one angle at the location where you can get a more distant background than others. This may allow you to use a smaller aperture without making the background too intrusive. The 150mm may well be better for this, if you've got the space.

Good luck, rather you than me!

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Link Posted 25/06/2010 - 14:01
This was on the 50-135. 75mm at f2.8 the door is about 25foot away from the subject and I think I was about 6 in front of it.

Even at 75mm and f2.8 the background is still quite discernable, If you want it softer than this then you need a longer focal length.

My Names Alan, and I'm a lensaholic.
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Link Posted 25/06/2010 - 14:13
More basic tips (further to Dan's comment):

Consciously check the edges of the frame before clicking! Check for cut off feet/hands, and whether there's anything obvious in the background (at one wedding there was a huge bright blue cover waaaay in the distance - but it still showed up! Quickly took another viewpoint )

Also, I always like to check for "phantom" hands - ie. hands that appear clutched around the partner's waist, or on the shoulder. Just tuck them slightly behind and it looks more natural. Watch out for bags as well - get them put down somewhere safe first

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Last Edited by MattMatic on 25/06/2010 - 14:13


Link Posted 25/06/2010 - 14:28
Please disregard my first entry in this thread. It was out of line and totally unnecessary.
Best regards

K20D, Optio I10, DA 18-55 1:3.5-5.6 AL II, A 1:1.7/50, D FA 1:2.8/100 Macro, Sigma 70-300 1:4-5.6 APO DG Macro, Pentax AF 360FGZ


Link Posted 25/06/2010 - 14:41
One thing to keep in mind if you want to use "long" lenses for full figure portrait: communication. If you stand too far from the subjects, it becomes very awkward to ask them to move one way or the other. You might want to use your cell phone to communicate.

I usually use the FA43 on APS-C camera for such occasion with great results. With longer lenses, communication starts to be problematic.
Yves (another one of those crazy Canucks)


Link Posted 25/06/2010 - 14:43
No worries Terry...

It was more trying to glean some wisdom from those with experience, I have a fairly firm grasp of the principles, just no experience of portraits (head or full)

I've told them that I'm not doing the full monty with all the arranged family & such, I just don't want the hassle. I'll concentrate mostly on the couple, then bridesmaids/best man included & only then on to all & sundry if time permits.

Looking back at our wedding shots, they are far too formal, so I'll try and interject some fun into the shoot. I just want to get some nice sharp & well exposed, full length shots in the bag first.

There is a small playground in one corner of the area so I'll be sure to keep that out of the background to avoid the bright colours. But I'm hoping to persuade them to hop on the swings & try a few fun shots.
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Link Posted 25/06/2010 - 14:51
It's lucky I don't have the cash in the bank, as this would be the perfect "excuse" to indulge my LBA & buy the 200mm f2
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