Manual Extension Tubes - Loss of contrast ?

[akazoly]

I bought a set of extension tubes on ebay:

I'm using the first (small) and the second (larger) tube on a Nikon 50mm f/1.8 lens.
These are manual extension tubes.

I took a photo of a lemon slice in a cup filled with water:

Did you notice the problem? I think the exposure is correct, I'm using an external light meter. I shoot RAW and increasing the exposure or decreasing it doesn't really help, but increasing the "blacks" and the "contrast" helps.

I took the photo on a cloudy day.
Do you have any idea what is the problem with blacks/contrast?

 

[16ale16]

well when using tubes like that in general ti happens that luminosity decreases. It means that if your lens is an f/2.8 it steps up to f/5.6 or similar (it depends on the tube).
Even contrast is reduced because you have to think that you're changing the optical scheme. So something that is optimized in a particular way, now is in another forced way. The light makes a longer journey to the sensor and it attenuates.
This behaviour is normal with tubes or duplicators, that's why I don't use them bit I prefer a digital zoom in the light room (with your preferred software), that in general is a little better, even if the bes solution ever is to use a macro lens

 

[akazoly]

Are you sure this problems happen with all extension tubes?

I took my photo at f/8 ; 1/80sec ; ISO 100

I perfectly understand there is a light loos with extension tubes, but I'm not sure about contrast.
The external light meter indicated 1/120sec, I estimated and set the camera to 1/80. I think the histogram looks good for me at 1/80:

I also took a photo at 1/40 which is brighter and has the same contrast problem.

BTW, I'm not using a lens hood. Is this a problem?
Or maybe the viewfinder let parasite light to pass on the image sensor?

 

[Overread16 4]

In my case I'd just increase backs, contrast and clarity and be happy with the result - you've no overexposure, though looking at the histogram you could probably overexpose from the meter reading a little more than you already are (you might get a few hotspots on the whites with this of course). The other factor is that an external meter won't be taking into account the aperture change that is occuring within the lens itself - try using the camera's built in meter to get a reading and see if it comes out more accurate from the start.

I'd also probably attribute some of the contrast loss to the fact that you're shooting through the glass container as well.

 

[Drake]

I don't have extension tubes, so I am not speaking from experience here, but I think that the cause of low contrast might be light leaks. If there's light coming any other way than through the glass, it will lower the contrast of photos. If the extension tubes have a reflective finish (smooth plastic), it's probably even worse.

 

[akazoly]

In my case I'd just increase backs, contrast and clarity and be happy with the result - you've no overexposure, though looking at the histogram you could probably overexpose from the meter reading a little more than you already are (you might get a few hotspots on the whites with this of course). The other factor is that an external meter won't be taking into account the aperture change that is occuring within the lens itself - try using the camera's built in meter to get a reading and see if it comes out more accurate from the start.

I'd also probably attribute some of the contrast loss to the fact that you're shooting through the glass container as well.

The Nikon D3100 can't meter through the lens.
I use manual extension tubes, there is no metering on the camera.
I set the aperture on the lens.

I'm using an old light meter to help me estimate the shutter speed. The light meter usually works well. LOL.

I'm also using a tripod. I compose the picture at f/1.8 and gently turn the aperture ring to f/8-f/16 (depending on the situation).

 

[Derrel]

It's basically just under-exposing...when you add the additional length of the tubes, the effective value of each lens aperture changes. I would disagree that the histogram looks good; it looks like it's an under-exposed frame!

 

[akazoly]

Adjusting the exposure doesn't make a difference (under or over exposure):

What do you mean in "the effective value of each lens aperture changes." ?
Thank you!

 

[Overread16 4]

Adding tubes chances the aperture of the lens - reducing the actual aperture that you're working at. So whilst the lens might have the aperture blades set at f1.8 the actual aperture could be a stop or two smaller than that reading. There is some maths out there for working out this reduction in aperture based on the amount of tube length added,but I don't have access to any on hand references to point you toward.

 

[Derrel]

What I mean is that when using the smallest ring (typically 11mm or so) and the second tube (typically 25mm), that means that there will be LESS light coming through each and every aperture setting.....when the lens is wide-open at f/1.8, the amount of light coming through will not be f/1.8 worth--it will be effectively, a LESSER amount of light.

If you have an external light meter, you would need to use a LOWER ISO/ASA value, to compensate for the longer extension of the lens from the film plane, caused by the extension tubes. This is called "bellows factor", or "extension factor".

 

[akazoly]

Thank you, Derrel. Can you check my RAW file to see if it's really an exposure problem, please: http://dl.dropbox.com/u/1924024/lemonslice2.NEF.zip

 

[Derrel]

Yes, it is an exposure issue; the original file needed more light....more exposure...either through a longer shutter speed, or more flash power, or a higher ISO value, or any combination of those three things.

 

[akazoly]

I think I have these tubes:

9mm tube (Tube 1)
16mm tube (Tube 2)
30mm tube (Tube 3)

I found this on a forum:

The loss of light can be calculated like focal length devided with the length of the tube equals the amount of f-stops times two.
fd= is f-stop difference
fl = focal length
l = lenght of extension tube
2 x fl / l = fd
That means for example that if the extension tube is 50mm long and you use 100mm lens you loose one f-stop.

In my case: 2x 50mm / 25mm = 4 (1 f-stop).
I loose 1 f-stop of light, altough I'm not 100% sure about this formula.

 

[Overread16 4]

About one stop sounds right - if you look at the histogram the larger vertical lines (on the screen shots you've posted) break the histogram into stops of light. You've very little to no light on the far right of around one stop in width so there is your stop of light that you can still expose for.

 

[djacobox372]

All extension tubes will perform the same--all they do is move the lens further away from the sensor, they have no glass inside them to change the image quality.