DOF Question

sunny_sony · Jul 7, 2011

Hi All,

Hope you all are doing great.

I have a basic question on DOF.

I like to shoot landscapes so like to have maximum DOF upto infinity.

I know that using the hyperfocal distance for F/8 at 18 mm (HYper Focal distance 6.7 Ft) I can get DOF from 3.35 Ft to Infinity with my Sony DSLR.

Now if 3.35 FT near distance is acceptable focus starting point for my photos upto infinity then can I shoot my landscapes on f/8 at 18mm using hyperfocal distance?

My Question is If we can get maximum DOF upto infinity using hyper focal distance Even at F/8 on 18 mm then why at all we need high aperture values like F/16 or F/22?

I know using F/16 or F/22 on 18 mm at hyperfocal distance we can get closer near distance focus but that's negligible compare to F/8 on 18 mm.

I would like to know if there's any other advanges of using high aperture value.

My Camera: SONY DSLR A390
Lens: 18-55 mm

Cheers

oldmacman · Jul 7, 2011

The smaller f-stops would be valuable if your lighting conditions change. There are times, in bright sunny days that you reach your maximum shutter speed with your ISO set to 100. Many of today's cameras support higher shutter speeds, but not everyone has the latest model. You can actually degrade your image quality if you go too small with your aperture setting. Your lens will have a sweet spot for sharpness and using the smaller setting is only advisable to adjust for light.

sunny_sony · Jul 7, 2011

Hi Thanks for the response. SO, which one has better image quality F/8 or F/22?

sunny_sony · Jul 7, 2011

Hi Thanks for the response. SO, which one has better image quality F/8 or F/22?

oldmacman · Jul 7, 2011

f8 would provide better image quality (depending on your lens). F8 may not be the sharpest aperture on your lens though.

sunny_sony · Jul 7, 2011

Can you please tell how can I find which is the sharpest aperture for my lens..

oldmacman · Jul 7, 2011

sunny_sony said:
Can you please tell how can I find which is the sharpest aperture for my lens..

The sweet spot will vary from lens to lens, but it will be somewhere in the middle. Take a few shots from f8-f13 and judge for yourself. If you search google on your lens, someone may have already done the test and be able to tell you.

Dao · Jul 7, 2011

First, for the Depth of Field (DoF) question.

DoF and Hyperfocal distance values are based on few factors. And those factors are related to Circle of Confusion (CoC) which definition as "In Photography, the circle of confusion diameter limit (“CoC&#8221

for the final image is often defined as the largest blur spot that will still be perceived by the human eye as a point." (Quote from Wikipedia) In other words, CoC kind of define if a particular spot in your photo is blur or not (or say, out of focus or not). And that is why DoF is related to the CoC.

The DoF value you obtain is based on the pre-defined CoC. In your case, the Sony DSLR is 0.019mm ( same as Nikon APS-C camera). And that value is calculated based on the size of your sensor, the size of the finial output (i.e. printed photo), no enlargement, and the viewing distance. And these are the factors affecting the DoF and Hyperfocal distance.

For example, if you are planning to print your photo on a Poster and the viewing distance is 2 feet. The CoC value is going to change, and as a result the DoF/hyperfocal distance also change. You may noticed some part of the photo is now out of focus (more blur). In this case, a smaller aperture MAY help.

But then again, if you view the Poster size photo at a longer distance, it will look fine.

As for sharpest aperture questions. In general, optical performance of the lens is the best when it is about 2 stops down from the max aperture. i.e. Canon EF 50mm f/1.8II lens, max aperture is f/1.8 and the sharpest aperture is somewhere around f/4 - f/8.

You can go to lens review sites and take a look at the lens's MTF chart for lens sharpest.

480sparky · Jul 7, 2011

DOF Calculator.

sunny_sony · Jul 7, 2011

I found this chart in sonylensreview.com

Sony DT 18-55mm f/3.5-5.6 SAL-1855 Center Sharpness

Focal Length / Aperture	f/3.5-4.5	f/5.6	f/8	f/11
18mm	Excellent	Excellent	Excellent	Excellent
28mm	Excellent	Excellent	Excellent	Excellent
55mm	n/a	Excellent	Excellent	Excellent

Sony DT 18-55mm f/3.5-5.6 SAL-1855 Border Sharpness

Focal Length / Aperture	f/3.5-4.5	f/5.6	f/8	f/11
18mm	Great	Excellent	Excellent	Great
28mm	Great	Great	Excellent	Great
55mm	n/a	Great	Great	Great

Does that mean F8 at 18mm is the sharpest on this lens 18-55mm.

Sorry I am new to photography so wanted to clarify this.

Dao · Jul 7, 2011

According to slrgear.com

"Wide open at 18mm and ƒ/3.5, the lens shows a generous area of sharpness reaching almost across the entire frame - around 1.5 blur units. There is some slight corner softness on the right side, and from one setting to the next this softness switches sides, leading us to assume our particular sample suffers from some de-centering. Stopping down to ƒ/4 shows negligible improvement, but at ƒ/5.6 and 18mm, the lens is very crisply sharp indeed, tack-sharp across the frame at 1 blur unit. It's sharp all the way to ƒ/11 at this setting, where diffraction limiting begins to set in, but even at ƒ/16 sharpness is only then beginning to reach 2 blur units."

So the lens at 18mm perform the best when stop down to f/5.6 - f/11

sunny_sony · Jul 7, 2011

Super like..Thanks very much...

Edsport · Jul 7, 2011

Another way to get max. DOP is to take 2 shots, one with the foreground focused and one with the background focused then combine the 2 shots or even 3 shots with mid point focused...