Outdoor Portrait? 85 1.8 or EF70-200f/4L IS USM on Crop Body 1.6x

[brando72]

Hi All. Love the forum and the masterful insight from many members.

I'm shooting a portrait outdoors ( warm light: about 1 hr before sunset) using a white background and 1 reflector. Equipment will be Canon 50d and a tripod. No flash. These shots will be very tight and only include head and top of shoulders on model.

Question: What lens will be sharper? The canon 85,, 1.8 USM or EF 70-200mm f/4L IS USM (IS will be shut off because using a tripod)

My sole intention is to get both eyes in focus and generally face but the rest does not really matter as much.

Can i shoot at 1.8 or should I stop to 2.8 for better sharpness? On the 70-200, if using the same focal length ( 85mm or 136 (crop fov) will the 70-200 be just as sharp at f4.0 or should i stop to 5.6?

Thanks for the time to read and any help is appreciated.

Best,

Brando72

 

[Loswr]

I'd use the 85mm f/1.8. First off, focal length won't matter if you're framing the shot the same, because if you do that, only aperture determines DoF (e.g. at 70mm f/4 or 200mm f/4 for the same head/shoulders framing, DoF will be the same).

I should point out that if you are using a canvas/muslin/paper backdrop, you don't need to shoot very wide - just wide enough to get the shutter speed you need (best at least 1/60 s for a posing model). The point of shooting wide open with a fast prime for a portrait is to blur out the background, but if you're artifically creating a clean, featureless background, no need to blur it much.

The 'sweet spot' for resolution with the 85/1.8 is f/4, but you'd have the flexibility to open it up a bit if you need more light (f/2.8 is not appreciably less sharp). The 'sweet spot' for the 70-200/4 IS is f/5.6, but again, you can go to f/4 with no meaningful loss. In their respective sweet spots, there's no appreciable difference in sharpness between the two lenses, although the 85/1.8 might have a slight advantage at the edges of the frame.

If you shoot tight at f/1.8 and want both eyes in focus, the model will need to be facing the camera - a bit of head angle will take one eye out of the DoF. Assuming you're about 6' away (approximate for a tight head shot), DoF at 85mm f/1.8 will be about 1.2". So, you're probably going to want to shoot in the f/3.2-f/4 range.

Hope that helps, and happy shooting!

 

[brando72]

Thank you very much neuroanatomist. Q: shutter at 1/60: if using a tripod, does shutter really make a difference between 1/30 or even 1/15. Or is 1/60 a sweet spot for negative camera movement. Also, for clarity and resolution sake, should I do everything possible to shoot at 100 ISO or lower if I can get that shutter sweet spot?

Many thanks.

 

[Loswr]

Well, the general guidance for handholding shutter speed is 1/focal length, and you need to take the crop factor into account for that. So, an 85mm lens on APS-C, without IS, you'd want at least 1/125 s shutter speed to get a good keeper rate without camera shake affecting your shots. Obviously, if on a tripod that doesn't apply - I do 30-60 s exposures on occasion.

But no, I wasn't referring to camera shake, since you already stated you'd be using a tripod. People move. Even someone holding still to pose for a shot will still have a degree of involuntary movement, and that small amount of motion blur will cost you sharpness. 1/60 s is generally sufficient to freeze the subject motion for a person holding still (you can go lower, and get decent results at 1/30 or even 1/25 s, but the keeper rate drops). At 1/15 s, you'll almost certainly have some subject motion in your shots.

As a side note, in addition to using a tripod, consider a remote shutter release (wired or infrared). In some situations, the self-timer works, but generally not so well for portraits where you'd probably rather click away.

Also, for clarity and resolution sake, should I do everything possible to shoot at 100 ISO or lower if I can get that shutter sweet spot?

You can't go lower than ISO 100 on your 50D. Even on a 5DII, which has an L expansion of ISO 50, that's just ISO 100 pulled down in exposure, so nothing is really gained.

But, the lower the ISO the better. Assuming you shoot in RAW you can likely go to ISO 400 with acceptable results. Try to keep it to ISO 100 if you can, but don't make your DoF too thin if ISO 200 or 400 allows you to use f/2.8, for example. Avoid the 'tweener' ISOs - stick to the full stops.

 

[brando72]

Once again, many thanks for your help!