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Should I Buy An f/2.8 Or f/4.0 Lens?

by Travis Lawton on 11/27/2011

Which Should I Buy

This is a question we hear quite often here at The Phoblographer. Both f-stops are very popular and are on many of todays most popular lenses. Of course if this was the only factor to the question, my answer would be hands-down the 2.8 since it is a full stop faster than the other. But it’s not quite that simple is it?

When people ask this question, many times they are asking about Canon’s 24-70mm f/2.8L (or Nikon’s 24-70mm f/2.8G version) lens vs Canon’s 24-105mm f/4L IS lens (or Nikon’s 24-120mm f/4G VR) lens. The biggest difference that pops out is the f/2.8 lenses do have that extra stop of light but the f/4 lenses both have image stabilization (or vibration reduction). Other factors that people ask about is how the bokeh is for each lens, what is the difference in reach look like, does the image stabilization counteract the f-stop difference, what is the difference when using a full frame vs cropped sensor camera, and how sharp one is against the other.

Since I’m a Canon guy, and I own a full frame camera and the EF 24-70mm f/2.8L lens, and my friend is letting me borrow his cropped sensor camera and EF 24-105mm f/4 IS lens, I figured this would be a great time to finally give people some answers to these questions.

Speed Vs. Stabilization

Lets start off talking about the elephant in the room about these two lenses. Being able to open your aperture from f/4.0 to f/2.8 is exactly one full stop of light however camera manufacturers will tell you that having a stabilization system in the lens will give you an extra 2-4 stops of light. What they mean is that you should be able to set your shutter speed much slower than you normally could if you were handholding the camera. By setting the shutter speed slower, it lets in more light,giving you extra stops of light.This is all relative to how shaky your hands are when holding the camera.

The stabilization can only do so much. I’ve been able to pull off usable handheld shots at around 1/10th or 1/15th of a second when that would be practically impossible (for me) without image stabilization.The following shot was made with my 24-70mm lens at 70mm, at the lenses maximum aperture of f/2.8 and since the sun had just gone done and I was losing light fast, I ramped up the ISO to 800 knowing full well my Canon 5D Mark II could handle that with no problems. With all of those settings plugged in, the camera said a shutter speed of 1/10th of a second would give me about a 2/3′s overexposed image, exactly what I wanted. As you can see, hand holding a 1/10th of a second shot produced a blurry image.

Canon EF 24-70mm f/2.8L: 70mm, 1/10th, f/2.8, ISO 800

Canon EF 24-70mm f/2.8L: 70mm, 1/10th, f/2.8, ISO 800

Next I changed lenses to the 24-105mm lens, set it to 70mm and dialed in the exact same settings except the aperture which I set at the maximum for that lens, f/4.0. As a result, the final image is 1 stop darker from the first but surprise surprise, it’s in focus. That is the image stabilization doing it’s job.

Canon EF 24-105mm f/4.0L IS: 70mm, 1/10th, f/4, ISO 800

Canon EF 24-105mm f/4.0L IS: 70mm, 1/10th, f/4, ISO 800

So now you’re saying the clear winner is the f/4 lens right? The difference between the two lenses is one stop of light but the stabilization gives you two to four stops extra right. Simple right? Wrong.

The stabilization helps prevent blurring images from camera shake but can’t do a thing for when your subject is the one moving. In the second image, if there were a person walking right past the cross walk sign (meaning they were in line with my focal plane), even though the stationary cross walk post would be clear, the person would be blurring because 1/10th is too slow to freeze a humans movements. If I were using the 2.8 lens in that same scenario, I would get the exact same exposed image but I could double my shutter speed to 1/20th. Still not ideal, but effectively twice as better.

Other than price, the speed vs. IS is probably the biggest factor that you could consider when looking at these two lenses. One thing that you should think about in terms of which to purchase is what camera you are going to use it on. More specifically how high that camera’s ISO can go and how well it can handle those higher ISO’s. My camera handles very high ISO’s (3200 and even 6400) so having an f/4 lens but knowing I can crank the ISO to compensate makes that lens a very real option.

Reach Difference And Full Frame Vs. Cropped Sensor

It’s right there in the specs of each lens, one just zooms farther than the other. In fact the 24-105 has exactly 1/3rd longer reach than the 24-70. Many people want the farthest reach they can get for the money and the 24-105 definitely hits both of those points.

One thing you need to know in the reach department is whether the camera has a full frame sensor or cropped sensor. A quick Google search will tell you the differences in detail between the two but the long and short (pun intended) of it is that a full frame sensor is a 1:1 ratio of the lenses reach. The cropped sensor is the one that has to be difficult, it has a 1:1.6 ratio meaning you have to times the numbers by 1.6. That means the 24-70mm lens on a cropped sensor is a 38-112mm and the 24-105mm lens becomes 38-168mm.

If you’re a little confused, it might help to see the difference. The images below are from a full frame sensor camera and a cropped sensor camera with both 24-70 and 24-105 lenses. Both cameras were set at the exact same spot and the subject not moved at all for each shot.

Full Frame Sensor With 24-70mm Lens

Full Frame Sensor With 24-70mm Lens

Full Frame Sensor With 24-105mm Lens

Full Frame Sensor With 24-105mm Lens

Cropped Sensor With 24-70mm Lens

Cropped Sensor With 24-70mm Lens

Cropped Sensor With 24-105mm Lens

Cropped Sensor With 24-105mm Lens

Bokeh

This is a very common question as everyone wants that sweet, sweet bokeh in their images. Have you been taking note of the bokeh in the previous images? If I hadn’t listed lens details under the images, could you have been able to tell which lens was which based solely on the bokeh? Well probably since the images are side by side. But that would be a different story if I only showed you one image at a time. What I’m using a lot of words to say is that the bokeh is quite acceptable for either one.

Take a look at the first two images again. The bokeh on the f/2.8 is obviously much better however the f/4 lens’ bokeh is also quite nice. Keep in mind that focal length factors into how the bokeh looks as well. Longer focal lengths make for better bokeh. That being said, that first image from the f/4 lens was taken at 70mm (to match that of the f/2.8 lens). Since this lens is capable of going to 105mm, the bokeh can be just that much better.

Let’s take a look at some bokeh’licious shots from the f/4 lens.

Full Frame Sensor, 105mm 1/30th f/4 ISO 800 Bounce Flash

Full Frame Sensor, 105mm 1/30th f/4 ISO 800 Bounce Flash

Full Frame Sensor, 105mm 1/15th handheld f/4 ISO 400

Full Frame Sensor, 105mm 1/15th handheld f/4 ISO 400

Cropped Sensor, 105mm 1/50th f/4 ISO 200

Cropped Sensor, 105mm 1/50th f/4 ISO 200

One thing to remember is that focal length plays into the bokeh equation. The further the reach of the lens, the better the bokeh tends to be. This helps in comparing these two lenses the f4 can reach much further than the f2.8. Personally, I do prefer the bokeh in the f2.8. It may not reach as far but it still has a full stop more bokeh power.

Sharpness

I feel these two lenses really stake up nicely to each other. Both are very sharp around f/4 and above. Yes, one of the lenses goes down to 2.8, so what I’m saying is that that lens isn’t it’s sharpest wide open. Going by the numbers, this is correct but I’m one of those photographers that doesn’t count sharpness as the end all be all of an image. I shoot wide open with that lens quite often and haven’t ever complained about the results. The affect is only very slight but if you look at the first image at f/2.8, you can see it is the slightest bit less sharp than the others.

These tests were performed using a Canon 5Dmk2 on a tripod, focused manually using Live View x10, at 70mm for both lenses, on aperture priority mode at ISO 100 and a 2 second timer to prevent camera shake. After the 24-70 lens shots were done, the 24-105 lens was swapped without moving the tripod or camera and the test apertures repeated.

EF 24-70mm f/2.8

EF 24-70mm f/2.8

EF 24-70mm f/4

EF 24-70mm f/4

EF 24-105mm f/4

EF 24-105mm f/4

EF 24-70mm f/5.6

EF 24-70mm f/5.6

EF 24-105mm f/5.6

EF 24-105mm f/5.6

EF 24-70mm f/8

EF 24-70mm f/8

EF 24-105mm f/8

EF 24-105mm f/8

EF 24-70mm f/11

EF 24-70mm f/11

EF 24-105mm f/11

EF 24-105mm f/11

EF 24-70mm f/16

EF 24-70mm f/16

EF 24-105mm f/16

EF 24-105mm f/16

EF 24-70mm f/22

EF 24-70mm f/22

EF 24-105mm f/22

EF 24-105mm f/22

Conclusion

Which lens should you buy? Well, I bought the 24-70 f/2.8, but that’s because what I shoot takes advantage of the speed of the lens more than the reach and the image stabilization. Basically, that is the question you have to ask yourself; what are you primarily going to shoot?

The Canon 24-70mm f/2.8L is better if you’re going to be shooting a lot of people. Sure image stabilization would be nice to have but like I said earlier, if your subject (the person) is moving, the stabilization won’t do anything but keep the background sharp. This applies to portraits, concerts, weddings, newborns, and especially kids. Kids are full of energy and are constantly moving, really, they never stop. You’ll benefit much greater from a faster shutter speed since you have an extra stop of light vs stabilizing the image.

The Canon 24-105mm f/4L IS lens is wonderful for still life photography as your subject is generally…still. This is a great lens to use as a walk-around lens to keep on your camera. The image stabilization will come in quite handy when photographing everyday things that catch your eye while walking down the street or if you want to take a shot of your meal in that poorly lit restaurant. The extra reach is definitely a plus toward this lens as well. It helps nicely if doing some more, shall we say, covert street photography. And you can’t forget that this lens is quite a bit cheaper. You can even get a kit of Canon’s amazing 5D Mark II Camera with the EF 24-105mm f/4 L IS USM Lens for even cheaper.

Don’t go on my genre suggestions for each lens as the end all be all answer on which to buy. They are both fantastic lenses and no matter which one you purchase let me be the first to say congratulations on money well spent.

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