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I have the Nikon D7500 which comes with the two kit lenses. I found that I did not get satisfactory bokeh when shooting full body images outside. My main aim is to be able to shoot full body portraits with some background and good bokeh.

I looked online and found that there are two options that people recommend which are the 35mm DX lens and the 50mm FX lens. I am confused between the two because I am seeing contradictory advice between them. Some say that the 50mm makes the field of view very narrow, while the 35mm images do not look as good. The advantage of having the 50mm lens would be it would allow upgradation to a FF camera but I might never do that as I am happy with my current camera. And I have also read that you can use the 35mm lens on FF cameras at low f-stops. Is that a good idea?

I am very confused regarding this and since I have been shooting for only a year, I dont think I can make a good enough judgement and wanted some help.

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    IMO, you are looking through the wrong end of the telescope. The question should be, "What's the ideal distance from the camera to the subject for a full-body portrait." Then it's a matter of finding the right focal length to make the subject fit the frame when you are standing at that distance from them. Q: how close can you stand to another person, and still be able to take in their whole body in a single glance? You probably don't want to be any closer than that for the portrait.
    – Solomon Slow
    Sep 3, 2020 at 13:16
  • Thank you. That is indeed the way I should be looking at it. I am thinking nothing more than 10 to 15 feet because I do not want to back away too much. But I am not sure how the field of view would be related to the distance between the subject and the camera. Is there an equation that I can use to work it out?
    – arun_kol
    Sep 4, 2020 at 1:05
  • "satisfactory bokeh" What is your definition for this ? Can you post an image of a portrait with satisfactory bokeh and one without ? IMO you don't generally need much background blurring to get good subject isolation and too much blurring removes context.
    – StephenG - Help Ukraine
    Sep 13, 2020 at 10:00

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Things to consider…

35mm in a DX body will let you take full body shots from reasonably close.
This will, however, give you 'short legs syndrome' if you're not careful. Shooting from any lower than chest height to try balance the leg length starts to get a bit 'up the nose'. Your bokeh will be harder to achieve on a relatively wide lens, even with the aperture wide open.

50mm will let you step back a bit, but you are now probably in 'must do this outdoors' territory in order to get sufficient distance between you, your subject & your background.

You can test both these ideas out first using your kit lens, just to see how the framing & leg length works, if not the bokeh.
Late edit I just realised you said 'two kit lenses', my bad - so you should be able to test this at all lengths & then know what you need for framing.
Add to that that your bokeh will get 'bigger' on a longer lens, so long as you can achieve the necessary distances.

Stepping up to an 85mm [or even longer] will improve your bokeh still further, and completely fix 'short legs', but you now need even more space to work in.
I'd be inclined to rent or borrow an 85mm, or 105mm or even a fast 70-200mm zoom before finally choosing which you need, or have space to use.
Personally, I tend to go for the longest lens I have room to step back for, for anything I need to separate subject from high blur background.

I guess one additional constraint might be budget. You can get a 35mm 1.8 for just over $£€ 100 if you shop carefully [it's nice enough but it's not a bokeh king, by a long shot], but the 1.4s go up in price pretty sharply - $£€ 400 for the 50mm up to $£€ 1,400 for the 85mm & nearly 2 grand for the 105mm.

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  • Thank you for the information. I was wondering how large will be the field of view behind the subject for this. Is there an optics equation that I can use to calculate how much of the background I can get in the frame based on the distances between the lens, subject and background and the lens information?
    – arun_kol
    Sep 4, 2020 at 1:08
  • There probably is, but maths isn't my strong suit;) You have all the potential lengths covered with your existing kit lenses, just dial them in & run some tests. Fast primes will change the optical quality but not the amount of frame you fill at any given distance. A 50mm indoors at home will likely back your subject into the opposite corner of the room to get full length, portrait [& they'll still have short legs]. Landscape you might need a bigger room. Then you really need at least the same or greater distance behind, so this quickly becomes either a studio or outdoor pursuit.
    – Tetsujin
    Sep 4, 2020 at 7:51
  • One upside is that the longer the lens you use, the narrower the aperture can be to achieve the same bokeh.
    – Tetsujin
    Sep 4, 2020 at 7:51
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Shooting full bodies with "satisfactory" bokeh is not easy. The diameter of your background blur is f/a where a is your aperture number. With full bodies filling the frame, you probably want bokeh circles for distant background that are at least 30mm in comparison to the body and/or facial features. Which is about 50mm/1.6. Shooting with 50mm on a DX sensor is possible, but you'll have to place your subjects at considerable distance to get them full-body, and the background at considerably more distance to give it sufficient blur. That can be difficult.

Full body shots with bokeh are more or less the quintessential use case for "full-frame with prime lens". You can labor around it a bit, but that requires lots of space for getting your scene set up and makes your subjects appear a bit flatter than if you work with an FX setup.

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  • The large focal length I use, the field of view gets smaller am I right? So is there a sweet spot for this?
    – arun_kol
    Sep 4, 2020 at 1:12
  • @arun_kol The "sweet spot" is determined by making it practical to set up the scene (mostly a camera restriction) and by having a "natural" perspective (independent of camera). From what distance would you usually take in people at full body height? Let's say you are at 4m of distance and want a 2m vertical section to appear with a height of 20mm (because you have an 18mm×24mm APS-C portrait frame). That gives you f=40mm. For the same kind of framing at f=50mm, you need 5m of distance, and at f=35mm, you need 3.5m of distance. That tells you about perspective but not bokeh.
    – user94588
    Sep 4, 2020 at 12:03
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50mm and back up a few feet. F4. Make sure your subjects have plenty of space behind them. Focus on the eyes or none of it will matter. Don't focus anywhere else.

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