focus plane ?

twinsmom

Another photographer and I have come to an impasse on this issue. I am in their workshop and do not agree with how they see this so can you help me to clarify.

If you are focused on a wall, will you lose sharpness at the edges? Meaning if I used a f1.6 and took a shot of graffiti would the edges be less sharp than the where I focused in the middle?
Thanks

Pono Photo

I would think yes, but I also think the lens in question needs more explanation. A nice clean 35mm or 50 mm would do it nicely, but the distance from said wall will also come into play. But if you get into a 15mm wide angle, you will lose sharpness. More factors come into play than just the aperture.

norm1

Focus plane

twinsmom wrote: »

Another photographer and I have come to an impasse on this issue. I am in their workshop and do not agree with how they see this so can you help me to clarify.

If you are focused on a wall, will you lose sharpness at the edges? Meaning if I used a f1.6 and took a shot of graffiti would the edges be less sharp than the where I focused in the middle?
Thanks

As I understand 'focus plane', this plane is not flat. I think of it as an arc . If you've focus on a spot say 15 feet in front of the camera, any spot on the arc at 15 feet in the lens' field of view will have the same focus. With a shallow depth of field created by the 1.6 aperature, the flat wall will fall out of the focus area as it gets away from the center of the shot. I hope this helps.

twinsmom

OK- that is what they said, but then how do you explain that you can take a macro shot and have something on the very edge that is on the same "plane" as the focus be just as sharp?

basflt

twinsmom wrote: »

OK- that is what they said, but then how do you explain that you can take a macro shot and have something on the very edge that is on the same "plane" as the focus be just as sharp?

because in Macro you dont use F1.6
F10 to F20 is more of use in Macro

if you d use a smaller aperture on the wall , it would all be sharp
( is also what @norm1 says )

zoomer

Yes.

When I take a picture of a group of people and focus in the middle I have to curve the ends of the group toward me so they are the same distance from the lens as the people in the middle.

ziggy53

"Field Curvature" is the name given to the problem. Most common and general photography lenses have some field curvature. Short focal length lenses and some zoom lenses seem to suffer the worst from field curvature.

Lenses designed for copying flat work are called "Flat Field" lenses because they are designed for very little field curvature. Flat field lenses are not necessarily good for general photography. Flat field lenses are often used for optical copier lenses, enlarger lenses and projection lenses.

Somewhat associated (in that it affects edge and corner sharpness) is chromatic aberration, where different frequencies of light focus at different angles on the image plane. Most general photography lenses are achromat designs, meaning that two frequencies will converge on the image plane. Apochromat (APO) designs should converge all visible light on the image plane, but that does not mean that all APO designs are sharper than all achromat designs.

Astigmatism and field curvature

Achromat

Apochromat

twinsmom

Thanks everyone.

Ziggy53- Thank you!!!! So if I'm getting this now. It's not really a focal plane issue. It's field curvature and or chromatic aberration. I really thought I had depth of field figured out then that came up and made me second guess what I thought I had understood. Time to go find a cool wall and play with different lens to check them out and how they handle this.

ziggy53

twinsmom wrote: »

Thanks everyone.

Ziggy53- Thank you!!!! So if I'm getting this now. It's not really a focal plane issue. It's field curvature and or chromatic aberration. I really thought I had depth of field figured out then that came up and made me second guess what I thought I had understood. Time to go find a cool wall and play with different lens to check them out and how they handle this.

Just so we're clear, many things can affect center of image to edge and corner image sharpness differences. Field curvature and chromatic aberrations are just a couple of the more common problems lens designers and lens manufacturers face.