Conclusions First
causticsadded clearly visible ripple-patterned light (caustic effects) to skin and the pool floor in 3 out of 3 images. The most stable keyword for underwater light expression.god raysgenerated diagonal rays of light streaming down from above in 2 out of 3 images. Distinct radial light shafts not seen in the control were confirmed.bubblesplaced large bubbles around the subject in 3 out of 3 images. The most reliable keyword for bubble expression.light beamsgenerates light shafts similar togod rays, but tends to also intensify the caustic pattern on the floor simultaneously.deep blueshifted the water color to a strong dark blue. The pool floor became nearly invisible, creating the impression of deep water.turquoisechanged the entire water to turquoise blue (a greenish bright blue), with the sandy floor also rendered brightly.hair floatingshowed no notable change in hair behavior compared to the control. The baseunderwaterprompt alone already produces flowing hair.
Experiment Design
| Item | Value |
|---|---|
| Model | z-image-turbo (6B, photorealism-focused distillation) |
| Steps | 8 |
| Sampler | euler |
| Scheduler | ddim_uniform |
| CFG | 1.0 |
| Size | 1024x1024 |
| Seed | 3 fixed (shared across all conditions) |
| Per condition | 3 images (3 seeds) |
| Total | 8 conditions = 24 images |
Base Prompt
Water-based base prompt with light and bubble modifiers added.
A00: control
| seed 1 | seed 2 | seed 3 |
|---|---|---|
 |  |  |
Observation: All 3 images show a woman swimming just beneath the water surface. Water surface ripple reflections are visible at the top. s1 has faint light patterns on the floor, s2 has a thin ripple pattern, and s3 has a grayish pool wall background with a few small bubbles on the left. Hair floats underwater in all 3. Water color ranges from bluish-gray to pale blue.
A01: caustics
| seed 1 | seed 2 | seed 3 |
|---|---|---|
 |  |  |
Observation: All 3 images show clearly projected mesh-like light patterns (caustics) on skin, swimsuit, and floor compared to the control. s1 shows large ripple-shaped light spreading across both the floor and the subject’s skin. s2 shows banded light patterns prominently on arms and legs. s3 shows caustics projected across the floor and the subject’s entire body, with clearly increased density and sharpness compared to the same seed in the control. Water color is essentially the same as the control.
A02: god rays
| seed 1 | seed 2 | seed 3 |
|---|---|---|
 |  |  |
Observation: Radial light shafts streaming diagonally from above were confirmed in 2 out of 3 images (s1, s2). s2 shows multiple clear light rays going from upper right to lower left. s1 also has light shafts projected onto the floor. s3 shows unclear rays with little difference from the control. Caustic patterns on the floor are similar to the control. No change in water color.
A03: bubbles
| seed 1 | seed 2 | seed 3 |
|---|---|---|
 |  |  |
Observation: Large bubbles appeared around the subject in all 3 images. While the control’s s3 had only a few faint bubbles, the bubbles condition produced spherical bubbles several mm to 1 cm in diameter lining up on both sides and above the subject in all seeds. s1 shows columns of bubbles rising on both sides of the subject. s2 has bubbles concentrated above the head and on the right side. s3 also has a column of bubbles on the left. The caustic effect on the floor is similar to the control, with no impact on light expression. Water color is slightly darker grayish-blue.
A04: light beams
| seed 1 | seed 2 | seed 3 |
|---|---|---|
 |  |  |
Observation: Caustic-style light patterns are visible on the floor in 2 out of 3 images (s1, s3), and multiple light shafts from above were confirmed in s3. s1 has ripple-shaped light spreading across the floor, giving the impression of more light than the control’s same seed. s2 shows little difference from the control. Compared to god rays, the light shafts appear more diffused, with a tendency to simultaneously strengthen the floor’s light patterns. No change in water color.
A05: deep blue
| seed 1 | seed 2 | seed 3 |
|---|---|---|
 |  |  |
Observation: The water color changed to deep navy to dark blue in all 3 images. While the floor was brightly visible in the control, in the deep blue condition the floor sinks into darkness, and the lower half of the subject also becomes slightly darker. s1 retains faint caustics on the floor but is overall dark. s2 has the sandy floor barely visible with the entire water a deep blue color. No change in light patterns or bubbles — purely the water color and brightness changed.
A06: turquoise
| seed 1 | seed 2 | seed 3 |
|---|---|---|
 |  |  |
Observation: The water color shifted to bright turquoise blue (greenish blue) in all 3 images. In contrast to deep blue, the sandy floor is depicted brightly and with transparency. s1 shows the water surface ripples in turquoise, with the floor also appearing blue-green. s2 is entirely bright turquoise with transparent-looking caustic patterns on the floor. Compared to the control, the water appears more transparent, giving it a look reminiscent of a shallow tropical sea. No change in light patterns or the presence of bubbles.
A07: hair floating
| seed 1 | seed 2 | seed 3 |
|---|---|---|
 |  |  |
Observation: No difference in hair floating behavior compared to the control. The underwater keyword already produces flowing underwater hair in the control, and adding hair floating produced no noticeable change in how the hair spreads or its direction. All 3 images show hair naturally spreading underwater, same as the control. s2 and s3 show a few bubbles, but the control’s s3 also had bubbles, so this cannot be counted as a difference. Water color is identical to the control.
Summary
| Keyword | Effect | Stability |
|---|---|---|
caustics | Adds ripple-patterned light to skin and floor | 3/3 |
god rays | Generates diagonal light shafts from above | 2/3 |
bubbles | Places large bubbles around the subject | 3/3 |
light beams | Light shafts + intensified floor light patterns | 2/3 |
deep blue | Changes water to deep dark blue, floor darkens | 3/3 |
turquoise | Changes water to bright turquoise blue | 3/3 |
hair floating | No difference from control | 0/3 |
For underwater light expression, caustics (ripple patterns) and god rays (light shafts) are effective. To add bubbles, bubbles reliably works. To control water color, deep blue (dark deep-sea tone) and turquoise (bright tropical tone) provide directional control.
hair floating is redundant with an underwater base — no point adding it. The underwater keyword alone produces naturally floating underwater hair.
![[Experiment] Prompts for Underwater Photography | 13 Conditions, 39 Images](/tips/underwater-photography-test/a01_underwater_s2_0_0000_7295072554507705269.webp)