Does zoom have effect on exposure?

[oleg_lempert2]

I often have to take separate exposure readings for my landscape shots. For example, one reading from bright sky,

the other from shadowed foreground. Sometimes I have to zoom to include either only sky portion or only foreground.

If I do not zoom I wouldn't be able to isolate that part of the scene. My meter mode is "evaluative" (Canon 30D).

If I were able to frame the same part of the scene without zooming, would the exposure values change?

Thanks

[lex_jenkins]

Yup, zooming around can influence exposure if you allow the camera to decide on metering automatically. TTL metering will respond to what it sees, even when the light falling on the scene remains constant. That's why reflected light metering isn't always the best choice. But if you don't have an incident meter, use your TTL meter to choose a specific exposure, lock it in manually and zoom away without fear of inconsistencies.

 

Unless the lighting changes. In which case, welcome to the joys of landscape photography, especially during the golden time when the light and color temperature changes every few minutes.

[photo_dark]

This is why I often stick to spot meter and take some readings before settling on an exposure.

 

Modern in-camera light meter algorithms seek to elimate blown out highlights (for the most part). Therefore, a scene with a large portion of bright sky and a dark foreground metered using matrix or evaluative will more often than not result in a darker exposure.

 

Zooming in (and thus lessening the bright sky in the composition) will likely result in a longer exposure to compensate for the change in readings.

 

When exposure is crucial, use your spot meter to determine the reading for the part of the scene you wish to be correctly exposed. Because of the more limited dynamic range of your digital camera, technically, there will only be one narrow band in the range of light that will be 'correctly' exposed in a scene. The rest is either underexposed or overexposed. Blow highlights or lost shadows are areas where the scene is over/underexposed to the point at which the sensor no longer get's enough light (or too much light) to render any details.

[phyrpowr]

Oleg,

 

correct me if I misunderstand your question.

 

If you fill the frame with a certain amount of light, regardless of whether you do it at, e.g., 35mm or 125mm, the exposure should be the same. So if you frame with a zoom at higher mm, or walk up to it and frame the same thing with lower, you should get the same EV.

[oleg_lempert2]

Jack. You understood my question. Your answer makes sense. Thanks.

[petemillis]

Jack, Oleg - not necessarilly. Look at most zoom lenses and they'll say something like "70-200mm f3.5/5.6" - the light getting through the lens to the film/sensor is reduced a lot when you use this sort of lens at maximum zoom. So if you retain the same ISO setting and the same shutter speed, the exposure will be much lower at maximum zoom than it will be at the widest end of the zoom.

[lovcom_photo]

Oleg, if you're using evaluative/matrix metering then why would you want to take readings from several parts of your composition? Better to take advantage of evaluative, AND work with the histogram too, to determine how much + or - exposure compensation is required. Shoot raw and expose to the right of the histogram to (1) minimize noise in the shadows, and (2) reveal more nuances/details in the shadows, and (3) extend the dynamic range a bit. Then during raw conversion, "normalize" the shadows, specify 16 or 32 bit color depth (to make room for future bit math rounding), and proRGB color space and you'll eek out the most image quality from your exposure.

 

As to taking separate readings, well since moving on to digital I find little or no reason to do this.

[savan_thongvanh]

it hasn't been said in so many words, so i will.

 

zoom, aka focal length, won't affect exposure in and of itself. But, on your lens, zoom may affect aperture,

which effects exposure.

 

plus the aforementioned spot metering comments...

[lovcom_photo]

Saven, actually zomming in will effect exposure. If you set the exposer at the widest focal length, then zoom in to the narrowest, the EV will need to be corrected. Narrow the FOV and less light comes in. Widen it and more light pours in.

[lex_jenkins]

Technically, the exposure value (using the specific term) should not change. But in actual practice it's likely to change. The problem is that changing the angle can change the reflected light. For example, if you stand at a distance and zoom in to meter a specific patch of ground, you'll get one reading. If you walk closer, use a shorter focal length and meter the same patch of ground it's likely you'll see a difference in metering even if the light falling on the subject hasn't changed at all.

 

Again, this is one of the tricky bits to metering. Even incident metering requires fairly careful alignment of the meter and light source, tho' it's a bit more forgiving than reflected light metering. And averaging or matrix metering tends to be a bit more forgiving of minor alignment error than spot metering.

 

Try it for yourself on a patch of even ground, such as green grass in sunlight. You'll probably see some differences between zooming in from a distance and one angle, vs. a shorter focal length and different angle.

[oleg_lempert2]

Savan,

I forgot to mention one important fact: I am in Aperture priority mode. So for both zoom instances, Av stays constant. I am sorry I failed to mention it in my original post.

Now, the correct question should be: will the Shutter Speed value change if I zoom ?

Thanks

[lovcom_photo]

Oleg, regardless if the camera is in Av, Tv, whatever, and regardless of the metering mode the camera is in, zooming in and out of a given composition will show different EV's at different focal lengths.

 

Doesn't the movie industry call this type of difference T-stops? Anybody care to comment on this? Lex?

[oleg_lempert2]

OK. I got it. Dan, thank you.

[Alex]

Oleg the answer to your last question is think is yes the shutter speed should change. Since you are reading

reflected light when you have a wider focal lenght there is more for the sensor to read (unless you are reading a

landscape without tonal differences) when you zoom in the light should change since you are eliminating part of the

landscape from the reading.

A least that is my interpretation.

Good luck

[lex_jenkins]

With a variable aperture zoom, yes, the shutter speed will change in auto mode. But this doesn't mean the EV is changing. As long as the subject, illumination and angle remain constant, the EV should remain constant. (Ignoring the other minor differences that can occur.)

 

For example, at EV 15 - typical daylight - and ISO 200, exposures can range from 1/2000 second @ f/5.6 to 1/125 sec. @ f/22. But it's still EV 15.

 

With a typical f/3.5-f/5.6 variable aperture zoom used wide open, the shutter speed must change to accommodate the change in effective maximum aperture as the zoom's focal length is changed. But the EV should, ideally, remain constant.

 

The variables are many, including the angle of reflected light, as I've mentioned, and variations in manufacturing. I've handled several lenses that show both measured and actual differences up to around 2/3 EV +/- between f/stops at center. Sometimes this is due to wear or manufacturing tolerances in manually operated aperture rings, but it can occur in auto exposure mode too. A notable lens with such discrepancies was one particular sample of the 20-35/2.8 AF Nikkor, which demonstrated some disturbing variations in actual exposures despite using the same EV with various shutter speeds and apertures. I expect that was just one lemon out of an otherwise respectable lens model.

 

And this doesn't even get into complications like light falloff at the corners and edges, the apparent effects of internal flare on the perception of exposure...

[petemillis]

Lex "And this doesn't even get into complications like light falloff at the corners and edges, the apparent effects of internal flare on the perception of exposure..." AND light fall off with distance - moisture in the air will cause light fall off - the more air the light has to travel through, the lower the intensity.

[alec_myers]

>>moisture in the air will cause light fall off

 

Actually I'm pretty sure if that were the case then the air in the air would cause light fall off too, and we'd all be living in our little blankets of darkness carrying lanterns with us because no sunlight would get through. We'd certainly never see the stars. It would also mean that the earth would look black from space and considerations of atmospheric change by adding CO2 would be somewhat different.

 

Dust and condensed water (mist/fog/cloud) does scatter light - but even condensed water vapour doesn't absorb *much* light, which is why all but the absolute densest clouds are pretty white.

[James G. Dainis]

Okay time to test it out. Set the camera to Av and aim it at a section of clear northern sky (equivalent to gray card.) Zoom in. Did the shutter speed change.? Now you know how your particular lens handles it.