F2 and dof

[donkeymusic]

Hi,

wrote this once and it didnt make sense, but if anyone can work out what i am trying to say and offer a solution it would be great.

F2 gives a shallow dof.

using a f2 allows me to open the aperture wide to allow more light in for darker situations.
if this is the case and shooting in a darker environment, do i still get the same dof?

Thanks

 

[Yellowbelly]

Yes. DOF is a function of focal length, sensor size and aperture.

 

[woof woof]

You mean... do you get the same DoF no matter what the lighting conditions?

If that's the question.... Yes.

You can easily test this by taking pictures in low light with long shutter speeds (so that the image comes out nice and bright despite the lack of light) and comparing them to shots taken in good light.

 

[mortimerhill]

Yes; depth of field depends on aperture, focal length and distance to the subject. If these are unchanged, then DoF is unchanged

 

[gad-westy]

Yes, no difference in terms of light however, depth of field doesn't just relate to the aperture but also the focal length and subject distance. It is possible to get a very large depth of field at F2 by either using a very wide angle lens and/or a subject some distance from the camera.

 

[woof woof]

Yes. DOF is a function of focal length, sensor size and aperture.

Couldn't resist saying... sensor size in itself (by itself) doesn't change the DoF. And now I'll run for cover

Sorry.

 

[Yellowbelly]

Couldn't resist saying... sensor size in itself (by itself) doesn't change the DoF. And now I'll run for cover

Sorry.

I'll forgive you just this once.

I don't think the OP is ready for that.

 

[Dale_tem]

You get the same dof regardless of light levels

f is the focal distance, so the depth of field is related to the f value, focal length and distance to subject.

35mm lens f/2 90cm to subject, the DOF is 5cm

35mm lens f/2 500cm to subject, the DOF is 166cm

200mm lens f/2.8 90 cm to subject, the DOF is .18cm

200mm lens f/2.8 500cm to subject, the DOF is 7 cm

 

[HoppyUK]

One point worth making is that cropping the image alters DoF, and all DoF tables assume the whole image is being used. When you crop, you are effectively changing the size of the sensor and that alters all the calculations.

Simple rule of thumb, if you crop 50% of the frame - that is, only use half the available sensor area - then DoF effectively increases by exactly one f/stop, ie f/2 delivers the same DoF as f/2.8, and so on.

 

[woof woof]

Well and very predictably said Hoppy

The DoF tables you love will contain some small print detailing some assumed things, CoC, image size and viewing distance etc. When you say that cropping an image changes the DoF I assume you're not talking about cropping an image in terms of cutting an image in half with a pair of scissors or masking off half of the sensor as to claim that this alone would alter the DoF would be a little You will be assuming that other things are altered at the same time as the crop, like the image magnification which would need to be increased as the crop factor increases if you want to produce the same sized print.

Instead of thinking about crop factors in terms of on line DoF tables and constant print sizes and viewing distances try thinking of it as throwing away the edges of an image because that is what it could be. What you could just be left with after cropping is a smaller image and all things being equal (I know they rarely are...) that smaller image would be exactly the same, DoF and all, as the equally sized section of the larger uncropped image. Alter the magnification at the same time as you crop (to maintain image size) and you will indeed alter the calculations just as you would if you altered the field of view or viewing distance drastically.

None of this is relevant in this instance. DoF is not affected by exposure or light level (other than being able to see the image ) and assuming the same focal length, focal point, camera to subject distance and f2 setting the DoF will be the same no matter what the shutter speed or ISO.

 

[EOS_JD]

RTather than saying focal length and distance to subject the correct way to say this is magnification.

DoF is dependant on 2 things. Aperture and magnification (how big the subject is on the sensor).