f/2.8 vs f/4 on sensor offering good results at high ISO

[yann_r.]

Hi there,<p align="justify">

 

Since I'm still comparing lenses and reading reviews in order to choose my next

Canon's DSLR gear (the body will surely be a 30D, not a full frame), I'd like to

know if it's really *useful* to own fast lens (f/2.8) whereas Canon's sensors

are very fine at ISO 1600. I'm very interested by the optical qualities of both

17-40 f/4L 70-200 f/4L.<br>

I'm also wondering about depth of field: is the difference really noticeable

between f/2.8 and f/4 aperture?<p align="justify">

 

Please note that I'm only an amateur looking for good quality, surely not a pro

looking for the top of the range. I know well that there's a BIG difference of

price. Let's just talk about technical/optical results, not about budget.<p

align="justify">

 

Thanks so much in advance for your sharp words.

[neild]

In terms of DOF, at the wide end (ie., 17mm) it will be difficult to tell f/2.8 and f/4 apart. At the long end (~200mm) I think there is quite a difference.

[bellavance]

I agree with Neil. Don't forget that 1.6 crop sensor cameras have more depth of field than full frame sensor models.

 

I was happy with the 24-70/2.8L on my 20D (on which it looked like 38-112) and I'm happier with the 24-105/4L IS on my 5D, to the point where I'm now thinking of selling the 24-70/2.8L.

 

I hope this helps.

 

Pierre

[yakim_peled1]

<ul>

<li> Wide aperture allows you to have shallower DoF.</li>

<li> Wide aperture allows you to have more background blur.</li>

<li> Wide aperture allows you to have higher shutter speeds. Important when your subject is moving.</li>

<li> Wide aperture allows you to have brighter viewfinder. </li>

<li> Wide aperture allows you to have faster AF. </li>

</ul>

<p> Whether or not these benefits are worth the added weight and cost is subjective. </p>

 

<p> Happy shooting, <br>

Yakim. </p>

[lovcom_photo]

Yakim is right...I want to add that the size of the sensor is irrelevent....we all would benefit from the fastest aperture, 1.6 or 1.3 or 1.0 crop.

 

Even the lowest noise 5D is better used at ISO 400, 200 and lower, so to do this one needs fast glass.

 

I am touring Italy's very dark and light challanged museums, the Vatican, etc, and the number one reason my images are coming out spectactular (no flash allowed) is because I'm using an F1.4 lens (Canon 35mm F1.4L), and ISO 400 to 800, and for the most DOF ISO1600 but this is the exception. If I had brought my F2.8 or slower lenses the image quality would be NOTICABELY worse.

[pjmeade]

Hello Yann, the background blur on the 70-200/f4 is very acceptable. I don't have a 2.8, so I can't compare like for like. I have no problems with pushing the 20D to 1600 or 3200. I know I'll see more noise, but if the worst comes to the worst, there's NeatImage.

 

But, I'm of the opinion that one should buy the best one can afford. So, if you can afford the 70-200/f2.8 (IS), go for it.

 

Hope this helps.

 

Pete

[robin_sibson1]

Yann, f/2.8 is only 'fast' in the context of zoom lenses, although it is true that the step up to f/2.8 is what triggers high-precision AF on the 20D/30D. A combination of medium-speed (f/4) zooms and truly fast primes, like the 50/1.4, might suit you better than going for f/2.8 zooms. I have quite a lot of lenses, more than I would ever consider carrying simultaneously, but my usual general-purpose carry-round kit for my 20D is 10~22, 24~105, 60/2.8, 135/2, and possibly Extender 1.4x. This delivers optical quality which is about as good as it gets, and provides a lot of flexibility including shallow DoF with the 135/2.

[george_lupton]

I have both the 17-40 and 70-200 f4 lenses and use them on a 5D. i find the optical quality of both to be excellent and if i'm honest can't see anything that could be done to improve the optical quality of the pictures they produce. in my opinion there comes a point when the performance of a lens gets to a place where to look for anything more would be a bit like chasing the holy grail - pointless. i think that these lenses 17-40 and 70-200L reach that quality threshold and for that reason would not spend the extra cash on the more expensive options.

 

i recently shot an up and coming band in a dark and dingy pub using the 70-200 f4L at iso 400 there was very little noise(none that i could notice)and my clients were delighted with the results.

[dcheung]

All of you forgot to mention that for better sharpness with your lenses, you need to stop down the aperture. Stopping a f2.8 lens to 4.0 still gives me decently narrow depth of field. However, starting with a f4.0 lens and stopping down to 5.6 starts to challenge the "decently narrow depth of field."

 

f2.8 lenses gives you the opportunity to either use them at 2.8 for low light situations where you can say get away with ISO800 rather than 1600 (a big difference in noise), OR, use them at f4.0 for shots that are much sharper than your f4.0 lenses at 4.0.

[yakim_peled1]

With many primes and L lenses it is not needed to stop them down to get sharp pictures.

 

Happy shooting,

Yakim.

[john_macpherson]

Pierre Bellavance Photo.net Patron, jun 29, 2006; 07:00 a.m.

"I agree with Neil. Don't forget that 1.6 crop sensor cameras have more depth of field than full frame sensor models."

 

Err.....am I missing something here - could you explain why this is so?

[salvatoresaitta]

17-40L is a great lens, I have it, it is sharp, solid, and has a reasonable price comparing with the 16-35L, absolutely a perfect choice ;)

[gluteal cleft]

Sometimes you're in a dim location, and can't (or shouldn't) use flash, and f/1.8 will get you there when f/2.8 won't, even at 1600. Whether that may apply to *you* is a legitimate question, but it does happen to me occasionally.

 

steve

[bellavance]

I can't explain why small sensor cameras have a lot more depth of field than full size sensor cameras, for a given magnification, but this is a fact.

 

My 5D has way less depth of field than my previous 20D, for the same picture. Look at small point-and-shoot cameras, such as my Canon S70, they have almost infinite depth of field.

 

Maybe someone reading this will be able to explain it to you. I've just found this with Google: http://tinyurl.com/fvybf

 

Pierre

[gdanmitchell]

Weiyang Liu wrote: "All of you forgot to mention that for better sharpness with your

lenses, you need to stop down the aperture..."

 

To which Yakim Peled replied: "With many primes and L lenses it is not needed to stop

them down to get sharp pictures."

 

These two statements are not contradictory.

 

Stopping down from the widest available aperture does produce "better sharpness." In

general, with the cameras we are considering in this forum something around f/8 (give or

take)

will likely provide the sharpest image obtainable from a given lens.

 

One can get "sharp pictures" (however you subjectively define "sharp") with larger

apertures

including, on some lenses, the largest aperture - they just won't be objectively as sharp as

those taken at the aperture that is optimum for sharpness. (Which is not to say that they

won't be great photos, won't benefit from DOF considerations, and won't be plenty sharp

enough for their intended use.)

 

There is a difference between "better sharpness" and "sharp," yes?

 

Dan

[awindsor]

Here on photo.net is Bob Atkins excellent article:

 

http://www.photo.net/learn/optics/dofdigital/

 

I am not certain of Yakim's assertion that fast lenses have faster AF. They certainly allow in more light and hence AF at lower light levels but none of my primes focus as fast as my f4 lenses. Lens design and AF motors have more to do with the AF speed than the maximum aperture.

 

My outdoor kit is 17-40/4L, 50/1.8, 70-200/4L, 300/4L, and 1.4x teleconvertor. Whereas I used to shoot using ISO 50 slide film I now will shoot quite happily at ISO 400. My indoor kit is 35/2, 50/1.8, and Tamron 90/2.8. I will replace the 90/2.8 by the 85/1.8 for the extra speed. I shoot jazz in very low light conditions.

 

The f4 versions are cheaper (a consideration) and lighter (for me a bigger consideration).

[yakim_peled1]

>> you subjectively define "sharp"

 

 

Indeed, "sharp" is a subjective term. To clarify, I'd say that any lens which I can't see the difference in A3 prints between wide open and stopped down is enough for me to define it as "sharp".

 

 

Happy shooting,

Yakim.

[yann_r.]

<p align="justify">Thank you so much all for your responses and for the interesting links you gave me. I see now better how aperture works with DOF. Regarding the sharpness of f/4 L lenses, I think that's a great one and I'm not sure to need faster lenses in a first time (it's quite rare I shoot in low light). Again, thank you.</p>