Shallow depth of field.

[bobrobert]

I have a 50mm prime 1.4 lens which I use for shallow depth of field. I am thinking of buying an 85mm 1.4 lens. Am I wasting good money if I do so? The question I have is if I fill the frame with a can of Coca Cola at 2 feet with my 50mm at 1.4 and there is a wall 10 feet behind and then fill the frame with a 85mm at 1.4 - I will have to be further from the can because the field of view is narrower - and the can is still 10 feet from the wall then what lens gives the shallower depth of field? I know about focal lengths and distances from subjects but not the relationship between the two lenses because of the different field of view. I know I can also crop the 50mm field of view. BTW I am tempted by the Samyang 85mm 1.4 which gets rave reviews. It is fully manual which I am happy with. I have the Samyang 24 mm 1.5 which I like.TIA

[PhotomanJohn]

Robert - If you compose the image the same with the 85mm that you did with the 50mm at 2 feel from the subject both at f//1.4 you will have the same depth of field of about 3/8". I am not sure about your reference to the wall 10 feet behind the can. In either case the wall will be greatly beyond the "depth of field" (defined as the area in acceptable focus). Maybe you could restate your question and what you are trying to achieve with the 85mm lens that you can't with the 50mm one so we can be of more help.

John

[Adrian]

I'm with John. It's hard to tell what benefit you are after. I have 50mm f1.4 and 85mm f1.2L. The 50mm is tricky for portraits as it can distort facial features (enlarged nose effect for example) , but you have mentioned cans of coke, not portraits. Unless you have a special reason for needing 85mm a lot I would say you are going to find limited opportunity to use both.

[arith]

I saw this strange thing in reference to a search for Helicon Filter :

[rawill]

Funny thing, I was going to suggest focus stacking to work around this issue.
Beat me to it.!

[rpcrowe]

I don't think that Robert is intending to shoot cans of Coke. I expect that was just an example. (forgive me if I am wrong).

Here is a real life scenario. Shooting a head and shoulders portrait with a 50mm lens on a full frame camera requires you to be approximately 5 feet (1.5 m.) from your subject. (Distances are from the Kodak Book, "set Up Your Home Studio", Planning The Space). Using an 85mm lens for the same head and shoulders portrait with a full frame camera, you need to be approximately 8.25 feet (4 m.) from your subject.

Shooting with the 50mm lens at f/1.4 at 5-feet will produce a total DOF of .27 feet or 3.24 inches...

Shooting with the 85mm lens at f/1.4 at 8.25-feet will provide a total DOF of .25 feet or 3.0 inches...

Granted, the DOF will be slightly different when using a crop camera but, the difference between the 50mm and 85mm DOF will be quite close on the crop camera also!

Now in my real life portrait shooting efforts, a DOF of around three or so inches is just too narrow and can cause one eye to be in focus while the other eye is OOF or the tip of the nose being OOF while the eyes are in focus or visa-versa.

I will shoot most of my portraiture using a 70-200mm f/4L IS lens wide open at somewhere around 135mm or more on my 1.6x crop Canon 7D camera. This will provide enough DOF to get both eyes and nose in focus but, have the focus falling off around the ears. IMO, the longer focal length is more flattering to people (especially females) than a shorter focal length such as 50mm. The drawback is that you need a larger space in which to shoot. However, if I did not have enough space to use a longer focal length lens, I would shoot outdoors rather than shoot indoors with a 50mm lens...



OTOH, if I desire, I can use this lens to completely obliterate the background details while still keeping my subject in focus; given a decent distance between my subject and the background



If I had framed this image approximately the same using a 50mm lens, I would have needed to shoot from a closer distance to fill the frame with my dog's head; and the length of her nose would have been exaggerated and look a lot longer than it does in this image. It is the lens to subject distance which controls the exaggeration of features, not the focal length. But, the focal length usually is the factor that determines the lens to subject distance...

Another advantage of the 70-200mm f/4L IS lens is that it shares rounded aperture blades technology with the EF 70-200mm f/2.8L IS II lens and produces very smooth bokeh...

[chauncey]

IMHO...most of canon's longer/high dollar glass will produce superior bokeh ( over 135mm).
Curious about bokeh comparing a 180 macro and a 300mm f/2.8 with extension tubes attached,
shooting the same subject at same FOV, I was hard pressed to discern any difference.
Yeah, it is comparing glass that you weren't asking about but, good bokeh is good bokeh.

[bobrobert]

Thanks for the replies. What I should have added is that I am going through a phase of shooting shallow depth of field with one part of the image sharp and the other exhibiting good bokeh. The can of coke was indeed an example. I don't do portraits. The Kodak reference was interesting and was what I was looking for. Obviously the bokeh differs with different lenses and the 10 foot reference was just an example of distance behind the subject. The further away the background the "blurrier" it gets. I like the match stick men effect.

http://www.flickr.com/photos/99408200@N05/9528087999/

http://www.flickr.com/photos/99408200@N05/9503804916/

These images were not taken with the above mentioned lenses but a Nikkor 70-200 mm f/2.8

It looks like I will be wasting my money because there is little difference between the lenses with respect to depth of field. The Samyang 24mm 1.5 hasn't a lot of difference between the 50mm but i bought it after buying the 50mm and can double as a cine lens.

[LocalHero1953]

I have a useful app to help me worry about depth of field. "Simple DOF" works on the iPad and, I guess on an iPhone as well - I haven't checked if android or windows versions exist. You dial in lens focal length, aperture and focus distance, and it gives you a neat graphic showing the limits of dof around the focal point, as well as the hyper focal length. You can play around with it for ever until you have the key numbers for your needs in your head.

[PhotomanJohn]

Robert - Thanks for clearing up what you meant in your original post. My confusion was that you only mentioned depth of field and never mentioned bokeh which is related to depth of field but actually a very different thing. Where depth of field for the same framed image doesn't change with focal length (assuming subject distance does not approach the hyper-focal distance for either lens) the bokeh changes significantly due to a couple of effects. First is the different perspective caused by the change in subject distance. With the longer lens/subject distance the objects in the background are larger with respect to the subject ("flatter perspective") which makes their being out of focus more obvious. Secondly is the design of the aperture blades in the lens whose shape effects the way the out of focus objects appear due to diffraction.

Experimenting is the best way to sort out how to get the effects you are looking for with the lenses you have available. Have fun.

John