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Lens Choices: Aperture vs. ISO
I am looking at the Canon Xsi, but prefer a longer zoom than comes in the kit. So I am considering a variety of zoom lenses (not necessarily Canon)..

The current SLRs have higher ISOs than those in the past, but the affordable zooms have smaller apertures..

I have a Panasonic Lumix DMC-FZ20 with a (built in) 12x lens (35 mm. equivalent 36-432) with an aperture of 2.8 throughout it's length..

Will the faster ISO make up for the smaller apertures (perhaps 3.5 - 5.6) in the new (IS) zooms?.

Should I always opt for the faster lens so I don't need to be concerned about noise from higher ISOs?.

Or am I asking the wrong question?.

Does anyone have recommendations for a lens that is comparable with the one on the Panasonic? My budget is not unlimited. Price IS a factor. (Or maybe I need to look at another SLR?)..

Comments (11)

Will the faster ISO make up for the smaller apertures (perhaps 3.5 -5.6) in the new (IS) zooms?.

Should I always opt for the faster lens so I don't need to beconcerned about noise from higher ISOs?.

They're related... but it's not precisely that simple..

Aside from the exposure issue, in which you're correct in that one can compensate for different aperture values by adjusting ISO (and/or shutter speed, depending on the scene and circumstances), there's also effects on DOF..

E.g. with a long telephoto, if it's f/2 (which is somewhat rare for long telephotos) or f/2.8 (more common, but not necessarily trivially priced), and you're actually taking advantage of that by shooting wide-open, the DOF will be very shallow. This may be what you want (and unlike exposure, DOF is not directly affected by ISO)..

But if you wanted more of the scene to seem sharp, you might instead be shooting at f/5.6 - f/8, in which case the fact that it -could- have been used at f/2 doesn't matter for exposure purposes. You'd have the DOF you want, and the viewfinder would still be bright and AF still fast (aperture is not closed down until exposure), but you'd be stuck needing the same ISO/shutter speed trade-off as with a lens that couldn't open up to f/2.8...

Comment #1

S. Brooks wrote:.

Should I always opt for the faster lens so I don't need to beconcerned about noise from higher ISOs?.

Or am I asking the wrong question?.

My approach is that the only reason for having a DSLR over a typical P&S is to take advantage of the versatility of the DSLR and the interchangeable lenses..

For that reason I want both high ISO and fast lenses. The combination makes things possible that you just can't have with only one or the other. If you opt for using only high ISO, you are missing half the fun..

Nothing is enough for the man to whom nothing is enough...

Comment #2

An excellent concise and accurate response..

JimOlympus E-510 and a bunch of stuff to hang on it...

Comment #3

There is another resource that can be chosen besides max lens aperture and ISO setting: exposure compensation. In fact I have a question that I was thinking of asking in my own thread, but it may be seriously relevant to the OP, so....

Does setting exposure compensation to shorten auto exposures by n stops have the same effect as bumping up ISO by n stops?.

On a film camera, the answer is no. Higher ISO means you loaded a grainier film, exposure compensation means you told the lab (yes, it could be you) to push the processing, which has a different downside (less vivid contrast or something, experts welcome here and elsewhere in this post)..

On a DSLR with a hard-wired image sensor these options are all just buttons and dials, but they might work in mysteriously different ways. Is there a difference between noise induced by high ISO settings in the camera and noise induced by ramping up the brightness/saturation in DPP etc?.

While on the subject of post processing, the freedom to take shots in full resolution raw mode and correct them later without having to wait an eternity between exposures is also a big vote-winner for DSLR over P&S..

Jeff.

I hate it when there are two four o'clocks in the same day - Terry Pratchett, Making Money...

Comment #4

All the above replies are good and on the whole, correct..

Another factor to take into consideration is that a lens with a larger aperture is USUALLY also better with regard to resolution and overall image quality and is OFTEN better built too..

Because this isn't NECESSARILY the case, it is worth looking at reviews and comparsions etc. before making your final decision about what to buy...

Comment #5

If money is a concern,.

Try large aperture (small f-number) Primes.Large aperture zooms are often "professional" and thus expensive...

Comment #6

Doubling the f-stop requires changing the ISO or exposure time by a factor of four. That's because exposure is inversely proportional to the square of f-stop..

So when weighing the choice of an f/2 vs f/4 lens, remember the ISO must be quadrupled to get the same wide-open exposure. The depth of field will be doubled for the same wide-open exposure (DOF is proportional to f-stop.)..

Comment #7

Trainspotter wrote:.

There is another resource that can be chosen besides max lensaperture and ISO setting: exposure compensation. In fact I have aquestion that I was thinking of asking in my own thread, but it maybe seriously relevant to the OP, so....

Does setting exposure compensation to shorten auto exposures by nstops have the same effect as bumping up ISO by n stops?.

...and after checking on the retouching forum the answer is no..

On a film camera, the answer is no. Higher ISO means you loaded agrainier film, exposure compensation means you told the lab (yes, itcould be you) to push the processing, which has a different downside(less vivid contrast or something, experts welcome here and elsewherein this post)..

... true for slide film, not so good for negatives. In any case with film the idea of switching rolls was sufficiently inconvenient to make post-processing seem like a lesser evil..

On a DSLR with a hard-wired image sensor these options are all justbuttons and dials, but they might work in mysteriously differentways. Is there a difference between noise induced by high ISOsettings in the camera and noise induced by ramping up thebrightness/saturation in DPP etc?.

... yes, for low ISO, pushing up the ISO gets you less noise than compensating the exposure down and correcting in pp. For higher ISO, the difference is minimal, but you still don't win..

Jeff.

I hate it when there are two four o'clocks in the same day - Terry Pratchett, Making Money...

Comment #8

Dave Martin wrote:.

Doubling the f-stop requires changing the ISO or exposure time by afactor of four. That's because exposure is inversely proportional tothe square of f-stop..

So when weighing the choice of an f/2 vs f/4 lens, remember the ISOmust be quadrupled to get the same wide-open exposure. The depth offield will be doubled for the same wide-open exposure (DOF isproportional to f-stop.).

Put more simply if you change exposure by two stops (e.g f/2.8 to f/5.6) then you need to change ISO by two stops (e.g IS0 400 to ISO 1600) or reduce shutter speed by two stops (e.g 1/400th becomes 1/100th)..

Common stop settings in ISO run:50, 100, 200, 400, 800, 1600, 3200..

Common f stops:.

F/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f11, f/16, f/22 (note that alternates numbers double as Dave said).

Common shutter speeds:1/30th, 1/60th, 1/125th, 1/250th, 1/500th, 1/1000th, 1/2000th.

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #9

Trainspotter wrote:.

There is another resource that can be chosen besides max lensaperture and ISO setting: exposure compensation. In fact I have aquestion that I was thinking of asking in my own thread, but it maybe seriously relevant to the OP, so....

Does setting exposure compensation to shorten auto exposures by nstops have the same effect as bumping up ISO by n stops?.

Shooting JPEG the answer is most definitely no..

Shooting RAW the theoretical answer is it depends on camera and processing software. On Nikon DSLRs such as the D80 noise increase with push processing in PP is linear to the increase from upping ISO in camera. Orther cams ad sensors may be different..

But the practical answer is it is not a good idea. Underexposure always creates noise in the shadows which is clearly visible when you brighten them. It requires skill and knowledge to remove it. Whereas bumping up the ISO the camera will sort the noise for you. If you know what you are doing you can improve what the camera does but at least then you have two options the first of which invloves the minimum use of time..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #10

Chris is right of course, simply increasing one parameter by a stop and decreasing the other by a stop is equivalent to the doubling/halving strategy I mentioned..

Unfortunately, manufacturers now subdivide f-stop and exposure time (but not ISO) by 1/2 or 1/3 stop, greatly confusing matters..

For many camera systems (essentially all P&S's), it is sufficient to count f-stop/time clicks: increasing f-stop one click implies decreasing exposure time by on click - the system designers will have made the clicks represent equal steps in exposure..

Unfortunately this isn't true for all cases, like DSLR's with their wide variety of manual, non-OEM lens possibilities. Also it doesn't carry over into ISO changes where only doubling/halving is the rule...

Comment #11

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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