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Photographic Tests: Sweet 35 optic

The whole Lensbaby concept isn't remotely about technical image quality, but instead giving a particular 'look' to the image. As a consequence, we'll be assessing the Sweet 35 in a somewhat different way than usual. So you won't find any charts or graphs on this page, just real-world examples.

Sweet Spot vs Aperture

The single most important characteristic of Lensbaby's optics, in terms of getting the image you want, is the dependence of the size and sharpness of the Sweet Spot with aperture. You can use a large aperture to keep attention tightly focused on a small area of the image, or stop down to make a larger area sharp. This is demonstrated in the rollover below; click on the F-number labels to download the full size image.

The Sweet Spot increases gradually in size on stopping down the aperture - at F2.5 it covers a small area of the center of the frame, but by F8 it's expanded to cover almost the height of the frame. By F22 the vast majority of the frame is quite sharp, with only the edges and corners obviously blurred.

It should come as no surprise to find that the Sweet 35 is also distinctly soft even in the center wide open, with a low-contrast, 'dreamy' look that's characteristic of spherical aberration. This clears up on stopping down; by F5.6 the lens is is genuinely sharp in the center of the sweet spot. At F22 the central region visibly softens due to diffraction, but this of course is balanced by a sharpening of peripheral areas.

Compared to 50mm Double Glass optic

Here we're looking at how the aperture-dependence of the Sweet 35's 'sweet spot' of sharp focus compares to Lensbaby's existing Double Glass (ca. 50mm F2) optic, using both APS-C and full frame cameras, using a profoundly boring but functional brick wall as the subject.

Double Glass, APS-C
F2.0
F4
F8
F16
Sweet 35, APS-C
F2.5
F4
F8
F16
Double Glass, Full Frame
F2.0
F4
F8
F16
Sweet 35, Full Frame
F2.5
F4
F8
F16

The Sweet 35 matches the characteristics of the double glass pretty closely, so if you already own and use this optic you should be able to adapt to the wider lens quite easily. Naturally, at any given aperture the sweet spot looks substantially smaller on full frame cameras compared to APS-C. Indeed at maximum aperture the sweet spot on full frame is very narrow indeed - you'll probably want to stop down quite a lot for most subjects to get an acceptably large zone of sharp focus.

To get the same size sweet spot on full frame as on APS-C, you need to use an aperture about 2 stops smaller. This brings with it some real operational difficulties, most notably a darker viewfinder (which makes focusing and composition less straightforward), and on some cameras less accurate metering. Both of these can be mitigated by switching to live view mode.

Blur and bokeh characteristics

One of the attractions of Lensbaby's optics so far has been the character of the blur outside the sweet spot, which tends to be pleasantly smooth and not distract from the main subject. The Sweet 35, by necessity, uses a more complex optical formula - four elements in a retrofocal design - and perhaps because of this, its blur characteristics tend to be a little bit more fussy than its stablemate's.

The examples below illustrate this: out-of-focus highlights are a characteristic elongated oval shape radiating away from the center of the frame. On the Sweet 35 these are somewhat more bright-edged than with the dual glass optic, and this is particularly pronounced on fullframe cameras.

Stopping the lens down decreases these highlights in sze, and on APS-C they tend to smoothen out quickly too, so even at moderate apertures (F5.6 or so) the blur becomes generally quite smooth and attractive. But on full frame it's not quite so simple, and you often have to stop down considerably if you want to soften point highlights. (Overall we just think the lens works a bit better on APS-C.)

Canon EOS 600D (APS-C) Canon EOS 5D (full frame)

Close-up / Macro

The Sweet 35 turns out to be a surprisingly useful lens for moderate close-ups. It focuses down to about 12" / 30cm, giving an image area of about 13 x 9 cm on APS-C (or 21 x 14cm on full frame). The peripheral blur can be used creatively to help focus attention on a specific subject, and reduce the distraction of in-focus elements towards the edge of the frame that you can't get with conventional lenses.

In these shots, both taken on APS-C format Canon DSLRs, the Lensbaby allowed blurring of the flowers towards the edge of the frame, which would be equally as sharp as the central area if shot with a conventional lens.

Coverage / Vignetting

One point you need to be aware of if you're planning on using the Sweet 35 optic on a full frame camera is that it can vignette quite significantly when used towards the extremes of tilt, which are needed if you want to place the sweet spot right at the edge of the frame. This vignetting decreases on focusing closer, suggesting that it's a limitation of the lens's image circle rather than physical vignetting by the barrel design.

This example illustrates the Sweet 35's vignetting on a full frame camera (the Canon EOS 5D) with a distant subject and extreme tilt.

Metering

The Lensbaby is to all intents and purposes a tilt lens, and like other tilt lenses it can cause all kinds of problems with the conventional TTL metering sensors used on SLRs (which measure light scattered off the focusing screen, and are calibrated for open-aperture metering with conventionally-centered lenses). Metering errors vary from scene to scene, but the more tilt you use, the more likely the exposure is to go awry. This means it makes sense to keep a close eye on your review histogram while you're working (and perhaps make liberal use of exposure bracketing).

When shooting the scene below (with a Canon EOS 1100D/Rebel T3), the initial metered exposure was far too bright, but after some experimentation a good exposure was achieved when the shutter speed was increased by two stops.

Metered exposure
-2EV exposure comp

One further quirk we noticed using Canon EOS bodies is that the cameras' conventional TTL metering will consistently overexpose at small apertures, even with the lens fully centered (by about a stop at F16). In practice this is more of a problem on full frame cameras than APS-C, simply because you'll probably want to use smaller apertures more frequently to expand the size of the sweet spot.

If you switch to Live View, of course, the camera meters using the imaging sensor directly, so these metering problems go away as if by magic. Live View can also give a better indication of where in the frame the sweet spot really is, and often allow more accurate focusing too. So it's well worth trying on cameras that offer it.

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